CHAPTER 6 Complex Patterns of Inheritance

Complex Patterns of InheritanceCHAPTER

6Specifi c Expectations In this chapter, you will learn how to . . .

• D1.1 analyze, on the basis of research, some of the social and ethical implications of research in genetics and genomics (6.3)

• D1.2 evaluate, on the basis of research, the importance of some recent contributions to the knowledge, techniques, and technologies related to genetic processes (6.3)

• D 2.1 use appropriate terminology related to genetic processes (6.1, 6.2, 6.3)

• D2.3 use the Punnett square method to solve basic genetics problems involving monohybrid crosses, incomplete dominance, codominance, dihybrid crosses, and sex-linked genes (6.1, 6.2)

• D3.3 explain the concepts of genotype, phenotype, dominance, incomplete dominance, codominance, recessiveness, and sex linkage according to Mendelian laws of inheritance (6.1, 6.2)

• D3.4 describe some genetic disorders caused by chromosomal abnormalities or other genetic mutations in terms of chromosomes aff ected, physical eff ects, and treatments (6.1, 6.2)

The inherited traits of an individual are the result of a complex array of genetic interactions. As genetics research continues to advance, we have a better understanding of these complexities. A signifi cant advancement is the Human Genome Project. In 2003, a team of over 2000 researchers, working in laboratory groups around the world, completed the Human Genome Project. For this project, numerous images like the one shown here were analyzed. Th is photo shows the products of chemical reactions that are used to identify the nucleotide sequence of a piece of DNA. Scientists used these to determine, base by base, the DNA sequence of the human genome.

Other goals of the Human Genome Project included identifying all of the human genes and making them available for study. Because such scientifi c goals have consequences for society, there are also groups of researchers that explore and monitor the ethical and social impacts of these scientifi c achievements.

240 MHR • Unit 2 Genetic Processes

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Launch Activity

Assembling a Mini-GenomeTh e 46 chromosomes that make up our genome contain over 3 000 000 000 base pairs. Each chemical reaction that is used to determine the sequence of DNA can only provide the sequence of a few hundred bases at a time. Th erefore, to determine the DNA sequence of the human genome, scientists all over the world worked together to analyze millions of DNA sequencing reactions. Th ey then assembled the DNA sequence of the human genome by piecing together the much smaller fragments of sequences. In this activity, you will model how scientists did this.

140T T TTTT T T TT T TT T TT TT T T TT TTA AAAA AAAA A A A A A AAA AA A A A AA A AAC C CCC CC C C CCC C C C C CCCG G G G G G G GG GGG

150 160 170 180 190 200 210 220

ultraviolet light

gelgel lanes

detector computer

output

The products of a DNA sequencing reaction are modified so they are visible under ultraviolet light. They are then separated in each lane of a gel-like material. The information in each lane is sent to a computer, which provides output in the form of a printout of the sequence of bases in a piece of DNA. Recall that nucleotides are often identified by their bases. For these data, red bands represent thymines, green bands represent adenines, blue bands represent cytosines, and black bands represent guanines.

Materials• paper DNA fragments• tape

Procedure 1. Obtain the sequence of DNA that you are to work with from

your teacher. 2. With your classmates, construct one continuous segment of

sequenced DNA from your individual fragments by matching overlapping sections and taping them into place.

Questions1. How did you decide how to match and link the fragments together? 2. How important was it to collaborate and discuss your results with

other class members in order to obtain the full sequence? 3. How important do you think it was for scientists to develop a

systematic and organized approach to sequencing the human genome? How do you think computers played a role?

Chapter 6 Complex Patterns of Inheritance • MHR 241

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CRCR

CRCW

CWCW

CR

CR

CR CW

CW

CW

gametes

red

white

pink

self-fertilization of F1 offspring

P generation F1 generation F2 generation

CRCWCRCR

CWCWCRCW

×

SECTION

6.1Beyond Mendel’s Observations of Inheritance

Key Terms

incomplete dominance

codominance

heterozygous advantage

continuous variation

polygenic trait

Much of today’s genetics research uses sophisticated technologies to study cellular processes at the level of individual molecules and atoms. In addition, international research collaborations and multi-million-dollar budgets are now common. Th ink of what a stark contrast this is to Mendel’s experiments. It is astounding that Mendel’s basic and, at times, simple observations led him to infer patterns of inheritance that still form the basis of our current understanding of heredity.

As more sophisticated experimental technologies became available, scientists realized that patterns of inheritance are more complicated than what Mendel proposed. Some patterns result in phenotypes that are between dominant and recessive phenotypes. Other patterns result in phenotypes that are created when both alleles for a trait are equally expressed.

Incomplete DominanceIncomplete dominance describes a condition in which neither of the two alleles for the same gene can completely conceal the presence of the other. As a result, a heterozygote exhibits a phenotype that is somewhere between a dominant phenotype and a recessive phenotype. One example is the fl ower colour of snapdragons (Antirrhinum majus). As you can see in Figure 6.1, a cross between a true-breeding red-fl owered plant and a true-breeding white-fl owered plant produces off spring with pink fl owers in the F1 generation. If the F1 plants are allowed to self-fertilize, the F2 generation will include off spring with all three phenotypes—red, pink, and white. Th e Punnett square in Figure 6.1 predicts that all three phenotypes will be observed in the F2 generation in a ratio of 1:2:1 (red:pink:white), which is what is observed experimentally. In true Mendelian inheritance, we would have predicted a phenotypic ratio of 3:1. Nevertheless, the alleles for fl ower colour do segregate according to Mendel’s law of independent assortment.

When representing incomplete dominance, upper-case and lower-case letters are not usually used to represent the alleles, since neither allele is dominant over the other. One way to represent incomplete dominance is by using superscripts. In the example of snapdragon fl ower colour, both alleles aff ect the colour of the fl ower, C. Th e two alleles are represented as superscripts, R for red (CR), and W for white (CW). Lower-case letters are only used to represent a recessive allele.

incomplete dominance a condition in which neither allele for a gene completely conceals the presence of the other; it results in intermediate expression of a trait

Figure 6.1 When red (C RC R) flowers and white (C WC W) flowers of the snapdragon are crossed, the resulting offspring have an intermediate phenotype, pink flowers (C RC W). In the F2 generation, all three phenotypes are observed.


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Incomplete Dominance and Human DiseaseTh ere are many examples of genetic disorders in humans that exhibit incomplete dominance. For example, there is a genetic disorder, called familial hyper-cholesterolemia, that prevents tissues from removing low-density lipoproteins (LDL) from the blood and causes very high levels of cholesterol in the bloodstream. In the majority of cases, the disorder is due to a mutation in the LDLR gene. LDL particles transport molecules like cholesterol throughout the body. Th e mutated version of the LDLR gene no longer produces a protein that interacts with LDL particles and removes them from the bloodstream. Th is disorder has an autosomal dominant inheritance pattern. So, an individual only requires one allele of the mutated form of the gene to show symptoms of the disorder. However, if the allele for the normal form of the gene is present, symptoms of the disease will not be as severe. People who are homozygous dominant for the trait have six times the normal amount of LDL in their blood and may have a heart attack by the age of 2. Heterozygotes have about twice as much cholesterol in their blood and may have a heart attack by the age of 35.

Scientists are now fi nding that identifying the patterns of inheritance for many traits is not as straightforward as fi rst thought. Today’s more accurate techniques are showing that, in some cases, what had been identifi ed as a dominant inheritance pattern may actually be incomplete dominance. As a result, an individual who is heterozygous for a trait is not exactly the same as an individual who is homozygous dominant for the trait.

CodominanceCodominance is a situation in which both alleles are fully expressed. A roan animal is an excellent, visible example of codominance. A roan animal is a heterozygote in which both the base colour and white are fully expressed. If you look closely at the individual hairs on a roan animal, such as the cow in Figure 6.2, you will see a mixture of red hairs and white hairs. One allele is expressed in the white hairs, and the other allele is expressed in the red hairs.

codominance the condition in which both alleles for a trait are equally expressed in a heterozygote; both alleles are dominant

Figure 6.2 A roan cow is the product of a mating between a red cow and a white cow. The red and white hairs may be present in patches, as shown here, or be completely intermingled.


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HbSHbS

sickle cellanemia

HbAHbSHbAHbSHbAHbA

sickle celltrait

sickle celltrait

sickle celltrait

sickle celltraitnormal

HbAHbA HbAHbS

HbAHbS

HbA

HbA HbS

HbS HbSHbS

Sickle Cell AnemiaSickle cell anemia is one of the most thoroughly studied genetic disorders. Although it is oft en described as being the result of autosomal recessive inheritance, it is actually an example of codominance. Sickle cell anemia is caused by a specifi c form of the gene that directs the synthesis of hemoglobin. Hemoglobin carries oxygen in the blood. Th e hemoglobin molecule that is made in individuals with the sickle cell allele leads to a C-shaped (or sickled) red blood cell. Th ese misshaped red blood cells, like the one shown in Figure 6.3, do not transport oxygen eff ectively because they cannot pass through small blood vessels. Th is leads to blockages and tissue damage.

Th e allele for normal hemoglobin is represented as Hb A, and the allele for sickle cell hemoglobin is represented as HbS. As shown in Figure 6.4, individuals who are homozygous (HbSHbS) have sickle cell anemia. Individuals who are heterozygous (Hb AHbS) have some normal and some sickled red blood cells. Th ese individuals are said to have the sickle cell trait, but they rarely experience any symptoms. In fact, having the sickle cell trait can be an advantage, because these heterozygotes are more resistant to malaria. Malaria is a life-threatening disease caused by a parasite that is transmitted to humans through mosquito bites. Th e parasite infects the liver and eventually the red blood cells. Th e sickling of red blood cells is thought to prevent the parasites from infecting the cells. Resistance to malaria is very benefi cial in certain parts of Africa, where deadly epidemics can occur. Th e sickle cell trait is an example of the principle of heterozygous advantage, which describes a situation in which heterozygous individuals have an advantage over both homozygous dominant and homozygous recessive individuals.

heterozygous advantage a survival benefit for individuals who inherit two different alleles for the same trait

1. Distinguish between incomplete dominance and codominance.

2. Why do geneticists use notations like CW and CR to describe incomplete or codominant alleles instead of using W and w or R and r ?

3. A plant that produces white fl owers is crossed with a plant that produces purple fl owers. Describe the phenotype of the off spring if the inheritance pattern for fl ower colour is

a. incomplete dominance b. codominance

4. Th e frequency of the appearance of the sickle cell allele in human populations is much higher in Africa than in most other areas of the world. What has been proposed to explain this observation?

5. Provide two pieces of evidence that support the idea that some inheritance patterns are more complex than those originally proposed by Mendel.

6. Scientists fi rst thought that sickle cell anemia was inherited as an autosomal recessive allele. What led them to identify the true inheritance pattern of the disease?

Learning Check

Figure 6.3 Normal red blood cells are flat and disk-shaped. Sickle-shaped cells are elongated and “C” shaped.

Figure 6.4 When a man and a woman are both heterozygous for the sickle cell gene, there is a one in four chance that they will have a child with sickle cell anemia.


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Possible alleles from female

Poss

ible

alle

les

from

mal

e

lA lB i

lA

lB

i

or

or

or or

lAlA lAlB lAi

lAlB lBlB lBi

lAi lBi ii

blood types A AB B O

Multiple AllelesTh e traits you have studied so far have all been controlled by one gene with two alleles, such as the fl ower colour in pea plants. Many traits in humans and other species are the result of the interaction of more than two alleles for one gene. A gene with more than two alleles is said to have multiple alleles. As you know, any individual has only two alleles for each gene—one allele on each homologous chromosome. However, many diff erent alleles for a gene can exist within the population as a whole.

Human Blood GroupsDo you know what blood type you are? In humans, a single gene determines a person’s ABO blood type. Th is gene determines what type of an antigen protein, if any, is attached to the cell membrane of red blood cells. An antigen protein is a molecule that stimulates the body’s immune system. Th e gene is designated I, and it has three common alleles: IA, IB, and i. As shown in Figure 6.5, the diff erent combinations of the three alleles produce four diff erent phenotypes, which are commonly called blood types A (IAIA homozygotes or IAi heterozygotes), B (IBIB homozygotes or IBi heterozygotes), AB (IAIB heterozygotes), and O (ii homozygotes). Th e IA allele is responsible for the presence of an A antigen on the red blood cells. Th e IB allele is responsible for the presence of the B antigen, and the i allele results in no antigen. Of the three alleles that determine blood type, one (i ) is recessive to the other two, and the other two (IA and IB) are codominant.

Rabbit Coat ColourAnother example of multiple alleles involves coat colour in rabbits, as shown in Figure 6.6. Th e gene that controls coat colour in rabbits has four alleles: agouti (C ), chinchilla (cch), Himalayan (ch), and albino (c). In that order, each allele is dominant to all the alleles that follow. Th e order of dominance sequence can be written as C > cch > ch > c, where the symbol > means is dominant to.

Figure 6.6 Rabbits have multiple alleles for coat colour, with four possible phenotypes.

Predict the possible genotypes for each rabbit.

Figure 6.5 Different combinations of the three I alleles result in four different blood types: type A,type B, type AB, and type O.

Himalayan

albino

agouti

chinchilla


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vby/vbyvb/vbvba/vbavf/vfvh/vhvl/vlv/v

Using a Punnett Square to Analyze Inheritance of Multiple Alleles

ProblemIf a man has type O blood and a woman has type B blood, what possible blood types could their children have? If this couple has six children, all with type B blood, what could you infer about the woman’s genotype?

What Is Required?You are asked to determine all possible blood types of the children and the possible genotype of the mother based on all the children having type B blood.

What Is Given?Th e man has blood type O, the woman has blood type B.

Plan Your Strategy Act on Your StrategyDetermine the possible genotypes of the man and the woman.

Since the man has blood type O, his genotype must be ii.Th e woman has blood type B, so her genotype could be either IBIB or IBi.

Make Punnett squares for all the possible combinations of genotypes.

motherIB IB

fatheri IBi IBi

i IBi IBi

motherIB i

fatheri IBi ii

i IBi ii

List all the possible genotypes and phenotypes of the children.

Th e children could have genotype IBi, resulting in type B blood, or genotype ii, resulting in type O blood.

What could be the mother’s genotype based on the children being type B?

Th e mother`s genotype is most likely IBIB.

Check Your SolutionTh e only genotype that produces type O blood is ii. To have type B blood, the woman must have at least one IB allele. Her second allele could be either IB or i. Since all of the children had to receive an i allele from their father, they must have inherited an IB allele from their mother. Since all of the children have type B blood, the mother is most likely IBIB.

Sample Problem

Clover Leaf PatternsTh e pattern on the leaves of the clover plant is also controlled by multiple alleles. While a single gene is responsible for clover leaf pattern, there are seven diff erent alleles for the pattern. Varying combinations of these result in 22 diff erent patterns that can be expressed in clover leaves. Patterns for the seven homozygous combinations of alleles are shown in Figure 6.7.

Figure 6.7 There are seven different alleles for clover leaf pattern.


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1. If a man has type AB blood and a woman has type A blood, what possible blood types could their children have?

2. A baby has blood type AB. If the baby’s mother has blood type B, what blood type(s) could the father have?

3. A couple just brought home their new baby from the hospital. Th ey begin to suspect that the hospital switched babies, and the baby they brought home is not theirs. Th ey check the hospital records, and fi nd that the man’s blood type is B, the woman’s blood type is AB, and the baby’s blood type is O. Explain why the parents are or are not justifi ed in their concern about this baby.

4. Four children have the following blood types: A, B, AB, and O. Could these children have the same two biological parents? Explain.

5. Some of the off spring of a chinchilla rabbit and a Himalayan rabbit are albino. What are the genotypes of the parents?

6. A chinchilla rabbit with genotype cchch is crossed with a Himalayan rabbit with genotype chc. What is the expected ratio of phenotypes among the off spring of this cross?

7. Could a mating between a chinchilla rabbit and an albino rabbit produce a Himalayan rabbit? Explain your reasoning. Your answer should include reference to the genotypes and phenotypes of the parents and the off spring.

8. In one family, all three siblings have type B blood. a. Use Punnett squares to show how two diff erent

sets of parent genotypes could produce this result. b. Which of the two sets of potential parents in your

answer to (a) is more likely to be the parents of these siblings? Explain why.

9. In dogs, coat colour is determined by the interaction between three alleles. Th e allele AS produces a dark coloured dog, a y produces a sandy coloured dog, and at produces a spotted dog. Th e order of dominance is AS > a y > at. Determine the following from the pedigree below.

a. the genotypes of the parents (I-1 and I-2) b. the probability of an off spring from the mating

between individuals II-2 and II-3 having spots c. the possible genotypes of individual II-1

21

I

II321

Key

darkcoloured

sandycoloured

spotted

10. A dark coloured dog is mated with a sandy coloured dog. Th e litter of puppies includes a dark puppy, a sandy puppy, and a spotted puppy. Use a Punnett square to determine the possible genotypes of the off spring and the parents. Note: Use the information about dog coat colour inheritance from question 9 to answer this question.

Practice Problems

Environmental Effects on Complex Patterns of InheritanceEnvironmental conditions oft en aff ect the expression of traits. For example, some genes are infl uenced by temperature. Th e dark colour in Himalayan rabbits, shown in Figure 6.8, is on the cooler parts of their bodies: the face, ears, tails, and feet. In these animals, dark colouring is the result of a gene that is only active below a certain temperature. One way to study the eff ect of the environment on expression of traits is to study genetically identical organisms placed in diff erent surroundings. For example, identical twins are genetically identical. Diff erences in the activity of their genes can be due to environmental eff ects.

SuggestedInvestigationPlan Your Own Investigation 6-A, Environmental Influences on the Production of Chlorophyll

Figure 6.8 The dark ears, nose, feet, and tails of Himalayan rabbits are thought to be caused by lower body temperature in these areas.


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0 1 2 3 4 5 6

Number of Dominant AllelesNumber of Dominant Alleles

Freq

uenc

yFr

eque

ncy

aabbcc AABBCC

AaBbCcaaBbCCAAbbCcAabbCCAABbccaaBBCcAaBBcc

AaBbccAabbCcaaBbCcAAbbccaaBBccaabbCC

aaBBCCAAbbCCAABBccAaBbCCAaBBCcAABbCc

AabbccaaBbccaabbCc

AaBBCCAABbCCAABBCc

Skin Colour

Polygenic InheritanceMendel carefully selected plants that had very diff erent heights so there would be no question about phenotypes. However, there are traits that exhibit continuous variation. Th ese are traits for which the phenotypes vary gradually from one extreme to another.

Some examples of traits that show continuous variation include height and skin colour in humans, ear length in corn, and kernel colour in wheat. Continuous traits cannot be placed into discrete categories because they vary over a continuum. For example, height in humans varies over a wide range of values. People cannot be categorized as only short or tall.

Traits that exhibit continuous variation are usually controlled by more than one gene. For some traits this can involve several genes. Traits that are controlled by many genes are called polygenic traits. A group of genes that all contribute to the same trait is called a polygene. Each dominant allele contributes to the trait. Recessive alleles do not contribute to the trait. For skin colour, the more dominant alleles a person has, the darker their skin. Th e graph in Figure 6.9 shows that there are more intermediate phenotypes than extreme phenotypes.

continuous variation a range of variation in one trait resulting from the activity of many genes

polygenic trait a trait that is controlled by more than one gene

Figure 6.9 This graph shows possible shades of skin colour from three of the sets of alleles that determine this trait.

Predict the effect of more gene pairs on the possible phenotypes.

A polygenic trait is one that is controlled by more than one gene and shows continuous variation. In this activity, you will choose one human trait that you hypothesize is controlled by more than one gene and shows continuous variation. You will then collect data from your classmates to test your hypothesis.

Materials• ruler or measuring tape (if necessary)• graph paper

Procedure1. In your group, choose one human trait that you think

is polygenic. Make sure your choice is one for which data can be easily and respectfully collected from your classmates.

2. Construct a data table to organize your data. Keep in mind that you will be measuring a particular trait and recording the number of times that measurement of the trait occurs.

3. Collect your data from your classmates.

4. Create a line graph of your data. Your graph should refl ect the actual measurements you took and the frequency of the values that you measured.

Questions1. Do your data support your hypothesis that the trait

you selected is polygenic? Explain.

2. How could this activity be improved to provide a clearer picture of the inheritance pattern of the trait you selected?

Activity 6.1 Identifying a Polygenic Trait


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tympanic membrane(or eardrum)

cochlea

Outer Ear Middle Ear Inner Ear

with BOB MCDONALD

Quirks &Quarks

THIS WEEK ON QUIRKS & QUARKS

S T S E

Related CareerHuman molecular geneticists study genetic processes in humans, particularly how genes function in human disease. These scientists are referred to as “molecular” geneticists because they look at the structure and function of genes at the molecular level. For example, they look for the eff ects of a genetic mutation by studying the mutant protein that is formed from it and how it aff ects processes in the body.

Selecting for Genetic DefectsMost scientists agree that certain inherited traits are favoured when they improve chances for survival. But what if improved chances for survival are due to a mutation associated with hereditary deafness? Bob McDonald interviewed Dr. David Kelsell, Professor of Human Molecular Genetics at Queen Mary College, University of London, to discuss this question.

Good News and Bad NewsScientists have known which gene is associated with most cases of hereditary deafness since 1996. A specifi c mutation in gene Cx26 (Connexin 26) is the culprit. People carrying one copy of the gene with the deafness mutation have normal hearing, while people with two copies are deaf. Because the deafness mutation in Cx26 is found in many human populations around the world, Dr. Kelsell‘s team suspected it must convey some kind of survival advantage.

Curiously, it does seem to. Individuals with the deafness mutation also had skin that was marginally thicker than the skin of people who do not have the mutation. Tests were conducted on the mutated skin cells to see whether the deafness mutation helped skin form a better barrier against bacterial invasions, and whether the aff ected skin cells healed diff erently. Results showed that the thicker skin could off er better protection and that healing could occur much more quickly. Therefore, while there is a risk of deafness if two copies of the mutation are inherited, one copy seems to provide better protection against skin diseases. As the Quirks host said, “How is it that one gene can aff ect two such diff erent and seemingly unrelated things—deafness and thickness of skin?” “This,” said Dr. Kelsell, “is one of the great mysteries.”

The protein product of the Cx26 gene is needed for movement of potassium ions between cells in the cochlea of the inner ear. This movement of ions is needed for proper hearing.

QUESTIONS

1. Is deafness due to a Cx26 mutation inherited by an autosomal recessive or autosomal dominant pattern? Explain your answer.

2. Explain why the deafness mutation of Cx26 is an example of heterozygous advantage.

3. Use the Internet or print resources to fi nd out more about the work of human molecular geneticists. What essential skills would you need in order to work in this fi eld?


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Section 6.1 R E V I E W

Review Questions 1. K/U A white-fl owered plant is crossed with a

red-fl owered plant. What is the likely mode of inheritance if the off spring produced are

a. plants with pink fl owers? b. plants with fl owers that are red and white spotted?

2. K/U Describe a human genetic disorder that results from incomplete dominance. Explain why it is classifi ed as incomplete dominance.

3. T/I In radishes, colour is controlled by two alleles that show incomplete dominance. When pure-breeding red radishes are crossed with pure-breeding white radishes, purple radishes are produced.

a. Provide the genotypes for the three colours of radishes.

b. What is the phenotypic ratio expected when two purple radishes are crossed?

4. T/I A farmer crosses a black rooster with a white hen. Of the seven off spring, three are black, three are speckled black and white, and one is white.

a. What can you infer about the inheritance patterns of the alleles for white and black feathers?

b. Given the inheritance pattern you described in part (a), what are the expected genotypes and phenotypes of the off spring produced by a cross between a speckled hen and a black rooster?

5. C Th e colour of an organism is controlled by one gene with two alleles: an allele that produces a blue colour and an allele that produces a yellow colour. Using genetic notations, describe the diff erences in genotypes and phenotypes of the organisms produced by crossing a true-breeding blue organism with a true-breeding yellow organism for the following three inheritance patterns. Use drawings in your answers.

a. blue is dominant over yellow b. blue and yellow are incompletely dominant c. blue and yellow are codominant

6. T/I Th e following pedigree shows the inheritance pattern of sickle cell anemia in a family. Known carriers of the sickle cell gene are noted. However, not all individuals have been tested for the sickle cell allele.

2

4

1

I

II

III

321

1

Key

normalphenotype,but not tested

sickle celltrait (carrier)

sickle cellanemia phenotype

a. Determine the genotype of each individual in the pedigree. If there are any you cannot be certain of, explain why.

b. Determine the probability that individuals II-3 and II-4 will have another child with sickle cell anemia.

7. T/I A chinchilla rabbit is crossed with a Himalayan rabbit, producing an albino rabbit.

a. Determine the genotypes of the parents. b. Identify other phenotypes expected from this cross

and give the predicted phenotypic ratios. 8. A Your friend has bred her female albino rabbit

with her male Himalayan rabbit. “I’m hoping I’ll get some agouti rabbits,” she says. What are her chances of getting an agouti rabbit? Explain.

9. C “Human ABO blood grouping is an example of the eff ects of multiple alleles, codominance, and dominance/recessiveness.” Use a table or graphic organizer to explain this statement.

10. A Siamese cats that spend their lives indoors tend to have lighter-coloured fur than Siamese cats that live outdoors. What genetic process could account for this change?

11. K/U What evidence is there that skin colour in humans is a polygenic trait?

Section Summary• Incomplete dominance leads to the expression of an

intermediate phenotype. In the case of codominance, both alleles are fully expressed.

• Although an individual has only two alleles for any gene, multiple alleles for a gene may exist within the population.

• Environmental conditions can infl uence the expression of certain traits.

• Polygenic traits are controlled by more than one gene and can usually be identifi ed by continuous variation in phenotype.


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purple flowers,long pollen







PPLL × ppll

PpLl

PL and pl gametes—more frequent

Pl and pL gametes—less frequent

Meiosis

GenotypePhenotype

Observe the phenotypesof the F1 offspring.

Allow the F2 offspringto self-fertilize.

Observe the phenotypesof the F2 offspring.

Cross a plant with purple flowers and long pollen to a plant with red flowers and round pollen.

: : :

×

×

F2 offspring having phenotypes of purple flowers, long pollen or red flowers, round pollen occurred more frequently than expected from Mendel’s law of independent assortment.

Red flowers,round pollen

purple flowers,round pollen

1.0


15.6

red flowers,long pollen

1.4

red flowers,round pollen

4.5

Fertilization

SECTION

6.2Inheritance of Linked Genes

As you have learned, there is no apparent interaction between non-homologous chromosomes during meiosis. Th e movement of each pair of homologous chromosomes is independent of the movement of other pairs of homologous chromosomes. Th is agrees with Mendel’s law of independent assortment. Recall that this law states that the alleles for a gene segregate independently of the alleles for other genes during gamete formation. However, Walter Sutton’s research showing that alleles segregate in the same way that homologous chromosomes do implies a very important point: alleles on the same chromosome do not assort independently. Th erefore, they do not follow the Mendelian inheritance patterns that have been discussed in this unit. It turns out that some genes are inherited together. Th erefore, some traits are oft en inherited together or are “linked.”

Linked GenesIn 1905, William Bateson and Reginald Punnett carried out the fi rst study that showed the movement of alleles that are on the same chromosome. Using sweet peas, they tracked the inheritance pattern of two traits: fl ower colour and pollen shape. Th ey knew that purple fl owers were dominant to white fl owers, and that long pollen shape was dominant to round pollen shape. Th eir results are shown in Figure 6.10. All four phenotypes that are predicted using a Punnett square were present in the F2 generation. However, there were far more of the phenotypes from the parental generation. Th is suggested that the gametes produced by the parental generation, PL and pl, tended to assort together rather than independently when producing the F2 off spring. Genes that do not assort independently are oft en called linked genes.

linked genes genes that are on the same chromosome and that tend to be inherited together

Key Terms

linked genes

sex-linked trait

Figure 6.10 A dihybrid cross between two sweet pea plants does not produce the expected phenotypic ratio of 9:3:3:1. These results support the theory that alleles on the same chromosome do not assort independently.

Identify Provide the genotypes of the F2 offspring.


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no crossing overduring meiosis

97%3%

recombinant gametes

crossing overduring meiosis

four types of gametes in unequal proportions

A

B

a

b

a

B

A

b

A

B

a

b

Crossing Over and the Inheritance of Linked GenesA chromosome may contain up to a few thousand genes. All of the genes on any one chromosome are called a linkage group because they tend to be inherited together. However, linked genes do not always stay linked—researchers have found that they segregate on a regular basis. Th is is due to the process of crossing over, which you learned about in Chapter 5. Recall that crossing over occurs in prophase I of meiosis, when non-sister chromatids exchange pieces of chromosomes.

Suppose you are studying two genes that are on the same chromosome and, therefore, linked. Crossing over between homologous chromosomes can occur. As shown in Figure 6.11, this will result in the alleles of the linked genes no longer being on the same chromosome. Th e alleles of the previously linked genes are now unlinked. Th is means that they will migrate into diff erent gametes. Th e result is that instead of two types of gametes being produced, four diff erent types of gametes will be produced in diff ering proportions. Th ere are fewer gametes with the recombined alleles because crossing over is a random event and it occurs infrequently.

Figure 6.11 In most of the gametes formed, there is no crossing over—they maintain the linkage of the alleles. In a small minority of gametes, crossing over occurs and alleles of previously linked genes become unlinked.

Describe why alleles of genes that are closer together on a chromosome are more likely to remain linked during meiosis.

Using Gene Linkage for Chromosome MappingScientists have discovered that alleles for a given pair of linked genes will separate with a predictable frequency and that this frequency is diff erent for diff erent pairs of linked genes. Th e frequency depends on how close the alleles of the linked genes are positioned on a chromosome. Crossing over occurs more frequently between alleles that are far apart on a chromosome than between alleles that are close together. Th erefore, a given pair of linked genes will separate more frequently than the alleles for another pair of linked genes if their alleles are farther apart on the chromosome. Th is process of determining the relative locations of genes on chromosomes is called chromosome mapping. Th ese types of linkage studies are useful for mapping chromosomes in species that reproduce rapidly and produce many off spring, such as plants and insects. But chromosome mapping is not useful in mapping human chromosomes. Chromosome mapping of humans only became possible when modern techniques that allow scientists to directly see the chromosomes became available.


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A

female male

Sex-linked InheritanceAn American biologist named Th omas Hunt Morgan, shown in Figure 6.12, originally did not accept Sutton’s chromosome theory of inheritance. In the early 1900s, Morgan chose to do research on the fruit fl y, Drosophila melanogaster, to develop a new and alternative theory. Morgan chose this organism because it is economical to maintain, reproduces rapidly, and has traits that are fairly easy to characterize. As Morgan collected data, however, his results soon convinced him that Sutton’s theories were correct. Nevertheless, Morgan’s meticulous research provided additional information about genetic inheritance.

In 1910, Morgan discovered an unusual white-eyed male among his fl y population. He crossed the white-eyed male with a normal red-eyed female. All the F1 generation had red eyes. Th is seemed to indicate that normal red eyes are dominant to the white-eye mutation. When Morgan crossed a male and female from the F1 generation, however, the results surprised him. All the females of the F2 generation had red eyes, half the F2 males had red eyes, and half the F2 males had white eyes. Th e discovery that the gene for eye colour was connected to gender led Morgan to conclude that the gene for eye colour is located on the X chromosome.

Like humans, female fruit fl ies have two X chromosomes, while males have one X chromosome and one Y chromosome. Th e fruit fl y F1 data indicated that the white-eye phenotype is recessive, since it was masked in all of the off spring in that generation. How did white eyes reappear in only the male fruit fl ies in F2, but remain masked in the female fl ies? Th e answer lies in the sex-linked genes—the genes that are located on the X and Y chromosomes.

Traits that are controlled by genes on either the X or Y chromosome are called sex-linked traits, because they are linked to the genes that determine sex. Th ey are identifi ed by their diff erent rates of appearance between males and females.

sex-linked trait a trait controlled by genes on the X or the Y chromosome

Figure 6.12 (A) Drosophila melanogaster traits that are often studied include eye colour and wing size and shape. Males and females can be easily identified. (B) Thomas Morgan’s ground-breaking research into the genetics of fruit flies was recognized in 1933, when he was awarded the Nobel Prize in physiology or medicine.

7. What are linked genes? 8. How are linked genes found experimentally? 9. What is chromosome mapping? How is gene linkage

used in chromosome mapping? 10. Suppose that two individuals with the genotype AaBb

are crossed, and the phenotypic ratio produced is about 3:1 (A_B_:aabb). Are the genes for the two traits linked? Explain.

11. Some traits are described as being due to sex-linked genes. Use your knowledge of chromosomes to explain what this means.

12. Many genetic tests are based on analyzing genes that are linked to alleles that cause disease. Explain how testing for a linked gene could lead to an incorrect diagnosis.

Learning Check

B


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XRXr

XR

Xr

red-eyedfemale

XRXR

white-eyedmale

XrY

XRXr

XR

XRY

Y

XRY

XRXR

XR

XR

F1 male

XRY

F1 female

XRXr

XRXr

Xr

XRY

Y

XrY

Sex-linked GenesTh e X and Y chromosomes, although paired together during meiosis and for karyotyping purposes, have very little homologous DNA. Th e X and Y chromosomes in humans have only a few genes in common. Th e human X chromosome is estimated to contain about 2000 genes, while the Y chromosome contains fewer than 100. Th e most important genes are the sex-determination genes. For all other genes on the X chromosome, females have two copies, while males have only one. Th is allows for the diff erence in the expression of traits for genes that are found on the X chromosome, which are oft en called X-linked genes. By comparison, only a few genes are known to be Y-linked, because there are signifi cantly fewer genes on the Y chromosome. When considering sex-linked traits, the allele on the sex chromosome is shown asa superscript to an X or a Y.

The Red and White Eyes of Fruit Flies

Red and white eyes were the fi rst sex-linked trait explored by Morgan. Th e possible genotypes and phenotypes in both males and females are listed in Table 6.1. XR indicates red eyes, which is the dominant phenotype, and Xr indicates white eyes, which is the recessive phenotype. Notice that female fl ies may be a carrier for the white-eye phenotype. However, if the allele for white eyes is present in males, it will always be expressed. Th is means that X-linked traits are exhibited more oft en in males. Punnett squares can be used to predict the outcome of crosses that involve sex-linked traits. Figure 6.13 represents some of the crosses that Morgan studied.

Table 6.1 Possible Genotypes and Phenotypes for Drosophila Eye Colour

Genotype Phenotype

XRXR Female with red eyes (homozygous dominant)

XRX r Female with red eyes (heterozygous, carrier for the white-eyed allele)

X rX r Female with white eyes (homozygous recessive)

XRY Male with red eyes

X rY Male with white eyes

Figure 6.13 In Morgan’s experiment on tracking the inheritance pattern of a sex-linked trait, the white-eye phenotype was passed from the father in the P generation through the daughter in the F1 generation.

Predict the genotype and phenotype ratios of the offspring created by crossing a white-eyed male and a heterozygous female.

SuggestedInvestigationThoughtLab Investigation 6-B, Sex-Linked Crosses


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I

II

III

XBXB = normal femaleXBXb = carrier femaleXbXb = CVD femaleXBY = normal maleXbY = CVD male

Key

XBY XBXb XBY

XBXB XBXbXBY

XBXB

XbY

XbY

XbXb

XbY

Sex-linked Traits in HumansSome examples of sex-linked traits in humans are listed in Table 6.2. As you can see, many are genetic disorders. If a disorder is X-linked dominant, aff ected males pass the allele only to daughters, who have a 100 percent chance of having the disorder. Females can pass an X-linked dominant allele to both sons and daughters, all of whom will have the disorder. Most sex-linked inherited traits in humans are X-linked recessive traits. Th erefore, while the male only needs to inherit one allele to be aff ected, the female must inherit both alleles to be aff ected. Th us, X-linked recessive traits aff ect more males than females in a family.

Table 6.2 Sex-linked Traits in Humans

Condition Inheritance Pattern Description

Red-green colour vision defi ciency (CVD)

X-linked recessive Cannot distinguish between certain shades of red and green

Duchenne muscular dystrophy

X-linked recessive Progressive weakening of muscles and loss of coordination

Hemophilia X-linked recessive Cannot produce a necessary blood clotting factor

Adrenoleukodystrophy X-linked recessive A build-up of fatty acids that causes progressive brain damage and death

X-linked severe combined immunodefi ciency (SCID)

X-linked recessive Decreased immune response due to low white blood cell counts

X-linked hypophosphatemia X-linked dominant Soft ening of bone, which leads to bone deformity

Hairy ears Y-linked Hair grows on the outside of the ears

Colour Vision Defi ciency: An X-linked Recessive TraitIn humans, there are inherited forms of colour vision defi ciency (CVD). Individuals aff ected by CVD have varying degrees of diffi culty distinguishing between diff erent colours or shades of colours. One form, called red-green CVD, is an X-linked recessive disorder. Individuals with red-green CVD have diffi culty distinguishing between shades of red and green. To track the inheritance patterns of sex-linked traits in humans, pedigrees are oft en used. Th e inheritance pattern of red-green CVD in one family is shown in Figure 6.14.

Figure 6.14 An X-linked recessive trait like CVD will affect more males than females in a family.


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Using Punnett Squares to Analyze Sex-linked Inheritance Patterns

ProblemDetermine the probability that a woman who is a carrier for hemophilia and a man without hemophilia will have a child with hemophilia.

What Is Required?You need to determine the possible genotypes and phenotypes of the off spring to determine if any of the children could have hemophilia.

What Is Given?You know the phenotypes of the parents, and you know that the pattern of inheritance is X-linked recessive.

Plan Your Strategy Act on Your StrategyAssign letters to represent each allele for the trait, and then determine the genotypes for the parents based on an X-linked recessive inheritance pattern.

Since the inheritance pattern is X-linked recessive, • let Xh = allele for hemophilia• let XH = allele for normal blood clottingTh e female is a carrier, so her genotype is XHXh.Th e male is unaff ected, so his genotype is XHY.

Use a Punnett square to predict the genotypes of the off spring.

femaleXH Xh

maleXH

Y

Complete the Punnett square. femaleXH Xh

maleXH XHXH XHXh

Y XHY XhY

Determine the predicted phenotypes of the off spring, and the probability of producing a child with hemophilia.

Th ere is a 25 percent chance of having a child with hemophilia (XhY). All other genotypes produce a child with normal blood clotting.

Check Your SolutionTo check your solution, ensure that the genotypes of the parents accurately represent the phenotypes, and that all possible combinations of gametes have been made.

Sample Problem

Hemophilia: A Common Sex-linked Trait in Humans Hemophilia is a condition that aff ects the body’s ability to produce proteins involved in blood clotting. People with hemophilia can suff er serious blood loss from simple cuts and bruises. Hemophilia is an X-linked recessive trait that aff ects more than 3000 individuals in Canada.

Hemophilia is oft en referred to as the royal disease because it spread among the royal families of Europe, through the descendents of Great Britain’s Queen Victoria, shown in Figure 6.15. Queen Victoria was a carrier who passed the allele on to some of her off spring. Arranged marriages among royalty of Europe were very common until the twentieth century. Pedigree analyses can trace the allele for hemophilia throughout the royal families of Spain, Russia, and Prussia.

Figure 6.15 Great Britain’s Queen Victoria was a carrier for hemophilia.


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Determining Sex-linked Inheritance Patterns in a Pedigree

ProblemTh e pedigree on the right shows the inheritance of red-green CVD in a family. Identify the genotype of each family member represented in the pedigree. How does the inheritance pattern in the pedigree support X-linked inheritance?

What Is Required?You need to determine the genotype of each individual and describe the evidence for X-linked inheritance.

What Is Given?You know that the pattern of inheritance is X-linked recessive, and you have the phenotype of each of the individuals (the pedigree).

Plan Your Strategy Act on Your StrategyAssign letters to represent each allele. Identify possible genotypes for each of the phenotypes based on an X-linked recessive inheritance pattern.

• let Xc = allele for CVD• let XC = allele for normal visionXCXC = unaff ected female XCY = unaff ected maleXCXc = female carrier XcY = male with CVDXcXc = female with CVD

Assign all possible genotypes, according to the information in the pedigree. • At this point, you cannot be certain of the genotypes for

individuals I-1, II-1, and II-3. Since they are unaff ected females, the possible genotypes are X CX c and X CX C.

I

II

III

2

1

21 3 4

1

2 3 4

?

? ?

XcY

XcY

XCY

XCYXCYXCY

XCY

Complete the pedigree with genotypes that you can infer based on the data in the pedigree.• You know that individual II-3 must be X CX c to produce

a son who has CVD. • Individual II-1 must be X CX c. Th e X chromosome she

received from her father is Xc and, since she is unaff ected, she must have received X CX c from her mother.

• You cannot be certain of the genotype of I-2 because both genotypes are possible (X CX c and X CX C), given the genotypes of the off spring.

I

II

III

2

1

21 3 4

1

2 3 4

XCYXCY

XCYXCYXCY

XCXc XCXc

XcY

XcY

?

Describe how the inheritance pattern supports X-linked recessive inheritance.

Th e allele for CVD is passed from the grandfather (I-1) through his unaff ected daughter (II-3) to her aff ected son (III-4). Th is pattern is indicative of X-linked recessive inheritance. As well, more males are aff ected than females, which also indicates X-linked recessive inheritance.

Check Your SolutionTo check the pedigree, ensure that all the off spring genotypes are possible given the genotypes of the parents.

Sample Problem

I

II

III

2

1

21 3 4

1

2 3 4


251-259_S62_BIO11.indd 257 18/05/10 1:35 PM

11. A woman who is a carrier for CVD and a man who has CVD decide to have children.

a. Determine the genotypes of these two people. b. What is the expected ratio of genotypes and

phenotypes among their children? 12. Th e mother and father of a boy who has CVD both

have normal colour vision. Use a Punnett square to explain how this can occur.

13. A woman with hemophilia and a man without hemophilia decide to have children. What is the probability that their sons will have hemophilia?

14. Nystagmus is a condition in which involuntary eye movement leads to poor vision. Th is condition is caused by an X-linked recessive allele. Suppose that a man and woman, both with normal vision, have two children. Th e boy is aff ected with nystagmus, and the girl is unaff ected.

a. Determine the genotype of the parents. b. Is it possible to determine the genotypes of the

children? Why or why not? 15. A woman has X-linked hypophosphatemia, which

aff ects bone development. She marries a man with normal bone structure. If the woman’s father also has normal bone structure, what is the probability that the woman and her husband will have a child with the disorder?

16. A true-breeding tan-bodied female fruit fl y is crossed with a yellow-bodied male. All of the off spring in F1 have tan bodies. In the F2 generation, all the females have tan bodies, 50 percent of the males have tan bodies, and 50 percent of the males have yellow bodies.

a. Describe the pattern of inheritance for body colour in fruit fl ies. Explain your answer.

b. Determine the genotypes of the fl ies described in the F2 generation.

c. What is the probability of producing tan off spring from a yellow female and a tan male?

17. Given the pedigree below, determine whether the pattern of inheritance of this trait is X-linked recessive, X-linked dominant, or Y-linked dominant. Explain your answer.

I

II

III

1 2 3 4

1 2

1 2 3 4

18. In one breed of dog, a mutant gene that causes hearing impairment is found on the Y chromosome. What are the possible phenotypes of off spring from each of the following crosses?

a. a male dog whose father is hearing impaired and a female dog whose father is not hearing impaired

b. a female dog whose father is hearing impaired and a male dog whose father is not hearing impaired

19. Suppose you have one homozygous dominant red-eyed female fl y and one white-eyed male fl y. What steps would you follow to produce a white-eyed female fl y?

20. Th e allele for short fi ngers is dominant to the allele for long fi ngers. What is the genotype of a male who has CVD and long fi ngers? If all of his children have normal vision and short fi ngers, what is the likely genotype of the children’s mother?

Practice Problems

Barr Bodies: Inactive X Chromosomes Since females carry two X chromosomes and males only one, why is there no diff erence in the expression of X-linked genes between males and females? Th e answer is that every cell has only one functioning X chromosome. In every female cell, one of the X chromosomes is inactive. Th e inactive X chromosome is condensed tightly into a structure known as a Barr body. At an early stage of embryonic development, one X chromosome in each cell is deactivated. Which X chromosome is deactivated can vary among cells. One visible eff ect of one X chromosome being inactive is the calico, or tortoiseshell, coat colour in cats, shown in Figure 6.16. In heterozygous females, roughly 50 percent of the cells have an active X chromosome with the allele for black coat colour, and 50 percent of the cells have an active X chromosome with the allele for orange coat colour. Th is results in a tortoiseshell coat with patches of both black and orange. Th e patches of white are the result of the interaction with a diff erent gene.

Figure 6.16 In cats, the alleles for black or orange coat are carried on the X chromosome.


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Review Questions 1. T/I Design an experimental procedure that you

could follow to determine whether two plant genes are linked.

2. K/U Describe how the process of crossing over of non-sister chromatids can aff ect linked genes.

3. K/U What experimental evidence would lead scientists to suspect that two genes are linked?

4. T/I A chromosome contains three genes, P, Q, and R. Th e percentage of gametes produced that have the genes separated due to crossing over is shown in the table below.Linked Genes

GenesGametes with

Unlinked Genes (%)

P and Q 5

P and R 18

Q and R 13

From these data, identify the gene pair with alleles that are closest together on the chromosome. Explain your answer.

5. C Draw a diagram that shows how crossing over can cause linked genes to become unlinked.

6. K/U List two features of Drosophila melanogaster that make this species a good choice for the study of sex-linked inheritance.

7. T/I A woman with regular vision and a man with regular vision have three children, one of whom has CVD.

a. What can you conclude about the genotypes of the parents?

b. What sex is the child who has CVD? How do you know?

8. K/U Describe the possible genotypes of the parents of a woman who has hemophilia. Explain your answer.

9. A Explain how a girl with Turner syndrome could have red-green CVD, even though both of her parents have normal vision.

10. T/I Th e following pedigree was given to a group of students to analyze. Th ey believe it indicates X-linked recessive inheritance. Do you agree or disagree? Explain your answer.

21

3 4

43

I

II

III5 6

65

1 2

21

11. K/U How do pedigrees for autosomal recessive traits and X-linked recessive traits diff er?

12. C A boy has Duchenne muscular dystrophy. His mother’s brother also has this disorder. Th e boy’s father and his two younger sisters do not appear to be aff ected by the disease. Draw a pedigree to illustrate the inheritance of Duchenne muscular dystrophy in this family. What is the probability that his sisters are carriers of the disease?

13. K/U Th e symptoms associated with X-linked dominant diseases are oft en more severe in males. Explain.

14. C Draw a sample pedigree to illustrate inheritance of hemophilia in a family. Make sure that your pedigree refl ects that particular inheritance pattern.

15. A Some women are heterozygous for an X-linked genetic disorder that results in a non-uniform distribution of sweat glands on their skin. Th ese women have patches of skin that lack sweat glands and patches of skin that have sweat glands. How can the Barr body cause this phenomenon?

Section Summary• Alleles of diff erent genes that are on the same

chromosome do not assort independently. Th ese genes are said to be linked and their associated traits tend to be inherited together.

• Sex-linked traits are expressed in diff erent ratios by male and female off spring because they are determined by the segregation of X and Y chromosomes.

• Although sex-linked genes are linked to the X and Y chromosomes, Punnett squares can be used to predict genotypes and phenotypes.


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the HumanGenome Project

is launched

ethical, legal, andsocial implications

(ELSI) programfounded

first US genomecentres established

rapid-data-releaseguidelines

established

Escherichia coligenome

sequenced

19971990 1991 1992 1995 1996

1865 1900 1913 1944 1953 1966

the first linearmap of genesis produced

the structureof DNA isdetermined

the genetic codeis identified

US Equal EmploymentOpportunity Commission

issues policy ongenetic discrimination

in the workplace

yeast(Saccharomyces

cerevisiae)genome sequenced

Gregor Mendeldiscovers laws of genetics

rediscoveryof Mendel’s work

DNA as the hereditary material is identified

y onmination

lace

cerevisiae)genome sequenced

A as the herematerial is identifie

1865

Gregor discoveof gene

theof Ddet

00 1913

the first linearmap of genes

roduced

coveryMendel’s

ork

SECTION

6.3The Future of Genetics Research

Key Terms

bioinformatics

genomics

genetic profi le

Genetics research is continually changing and developing in response to new discoveries. Many genetics researchers now focus on obtaining more and more detailed information. In addition to wanting to know the sequences of genes that are associated with certain inherited traits, investigators want to know how those genes play a role in determining those particular traits. Looking for answers to these types of questions has led to the development of more sophisticated technologies and equipment, and has resulted in new scientifi c fi elds of study. In addition, many studies now require the collaboration of scientists from very diff erent disciplines, such as biology, chemistry, physics, sociology, bioethics, and political science.

The Human Genome ProjectIn the opener for this chapter, you were introduced to the the Human Genome Project. Determining the DNA sequence of the human genome is considered to be one of the most pivotal contributions to science ever made. Nevertheless, achieving this scientifi c landmark depended on many discoveries that came before it. Figure 6.17 highlights only a small number of developments since Mendel’s work that formed the foundations of this project.

An important component of the 13-year Human Genome Project was determining the DNA sequences of other organisms. Th is allows scientists to make comparisons between species and learn even more about important features of genomes. Overall, identifying the genome sequences of humans and many other organisms allows for a much more comprehensive understanding of biological systems. Th is knowledge will have a wide range of applications in fi elds such as human health, agriculture, and the environment.

Figure 6.17 The Human Genome Project achieved many milestones and has provided a springboard for decades of future research. Nevertheless, this project would not have been possible without several essential preceding discoveries—including Mendel’s studies of pea plants.


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free access to genome information established

fruit fly (Drosophila melanogaster)

genome sequenced

draft versionof human

genome sequencepublished

mustard cress(Arabidopsis thaliana)genome sequenced

1999 2000 2001 2002 2003

1972 1977 1983 1989 1990 2003

sequences of mouse, rat, andrice genomes completed

human genomesequence

completed

full-scale humangenome sequencing begins

sequence of first humanchromosome

(chromosome 22)completed

methods for determining the sequence of DNA are developed

cystic fibrosisgene is identified

first human disease gene—for Huntingtondisease—is mapped

recombinantDNA technologyis developed

HumanGenome Project

(sequence of first human

chromosome(chromosome 22)

completedsoe

staope

ard cresspsis thaliana)sequenced

ce

sequences of mouse, rat, anrice genomes completed

reDNis

1983 1989

first human diseasegene—for Huntington

What’s in Our Genome?In addition to determining the actual sequence of the nucleotides in the human genome, scientists had to make sense of the sequence. Trying to make sense of the sequence can be compared to reading a book written in a language nobody knows or understands. Imagine the genome as words in a book written without capitalization, punctuation, or breaks between words, sentences, or paragraphs. Also, suppose there are strings of additional letters scattered randomly between and within sentences. Figure 6.18 shows how a page from such a book might look. To understand what is written, you have to decode the jumbled text. Similarly, scientists had to decode the sequence of our DNA to learn about the human genome. When the Human Genome Project began, there was a great deal that was not known about our genome. For example, it was not known how many genes humans actually had and how much of our DNA is part of those genes.

Aft er sequencing the entire human genome, scientists observed many things that surprised them. Some of these discoveries include the following:• Only about 2 percent of the nucleotides in the human genome make up our genes

and code for all the proteins in the body.• Th e estimated 25 000 total number of genes is much less than scientists predicted.

Previous estimates were between 80 000 and 140 000.• Over 50 percent of our DNA consists of stretches of repeating sequences.• Th ere is very little genetic variation within our species. About 99.9 percent of the DNA

sequence is almost exactly the same in all people.Having the sequence of the human genome only represents a starting point. It is

like being given the pages of an instruction manual for the human body. Th e next steps involve fi guring out how to interpret all of the information and use that to understand how everything works together. Scientists agree that this process will take many more years of research.

Figure 6.18 Decoding the DNA sequence of the human genome is like figuring out where the punctuation and capitalization must go to understand what is written on this page.


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The Development of BioinformaticsIn the Launch Activity at the beginning of this chapter, you simulated the work required to piece together the sequence of a small fragment of DNA. Imagine doing this work by hand for the over three billion base pairs of the human genome. Sequencing the human genome and the genomes of other organisms generated exceptionally large amounts of data that needed to be organized and shared among labs around the world. A new fi eld of study, called bioinformatics, arose from this need. Bioinformatics is a branch of biology that deals with applying computer technology to create and maintain databases of information that can be analyzed to better understand biological processes.

Bioinformatics is a relatively new branch of biology. American chemist Margaret Dayhoff , shown in Figure 6.19, is the founder of bioinformatics. Her work, which began in the late 1940s, involved creating a computerized protein and DNA sequence database—the fi rst bioinformatics project. Today’s bioinformatics exists because of simultaneous advances in three areas: techniques to sequence biological molecules such as DNA and proteins, computer database soft ware to sort and store massive amounts of genetic information, and communication technology to share information around the world effi ciently. Today, there are many on-line genetics databases available that allow easy access to vast amounts of genetic information by all members of the public—not just scientifi c researchers.

Bioinformatics is just one of a number of newly developed fi elds, all of which involve using computers to study biological problems. For example, computational biology involves developing mathematical models and computer simulations of biological processes.

bioinformatics a field of study that deals with using computer technology to create and analyze large databases of information

In this activity, you will join the worldwide community of scientists who explore information stored in the many on-line databases that are available.

Materials• computer with Internet access

Procedure1. Choose one of the genetic disorders provided by your

teacher.

2. Use the Internet to access the website that you will be using. Your teacher will provide a demonstration to help you get started.

3. Spend some time looking at the diff erent databases that are available from this site. What diff erent types of information about a genetic disorder can be obtained from them?

4. Choose three or four types of information that are available about the genetic disorder that you selected.

Questions1. Summarize the information you collected on the genetic

disorder you investigated.

2. Based on your experience with the on-line databases, what was the most eff ective way of obtaining the information that you were looking for?

Activity 6.2 Accessing Genetic Information

Figure 6.19 A chemist named Margaret Dayhoff is considered to be the founder of bioinformatics.

13. What were some achievements of the Human Genome Project?

14. How much of the human genome is actually used to code for proteins?

15. List three types of technologies that contribute to developing the tools used in bioinformatics.

16. Explain how bioinformatics contributed to the Human Genome Project.

17. Why was development of the Internet crucial for the Human Genome Project?

18. Describe an experiment that requires bioinformatics.

Learning Check


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C G T T C T C T A T T A A C A .. .G C A A G A G A T A A T T G T .. .

three billion DNA base pairsin the cell nucleus

thousands of different proteins are produced in trillions of cells

phenotypesare expressed

Figure 6.20 Genomics is the study of how an organism’s genome contributes to its phenotype.

Genomics: The Study of GenomesJust as genetics is the study of genes, genomics is the study of genomes and how genes work together to control phenotype, as illustrated in Figure 6.20. Although some traits are determined by only one gene, most traits involve multiple genes. To understand how an individual gene produces a specifi c phenotype, researchers such as Mendel and Morgan chose one gene and studied it and its phenotype across many individuals. A signifi cant advantage that came from the Human Genome Project was the ability to consider multiple genes and the genome as a whole. Th is allows scientists to study the interactions among many genes and how they all contribute to a phenotype. Computer technology and fi elds such as bioinformatics play a vital role in this by allowing scientists to analyze large amounts of information from a variety of sources.

Although there is considered to be little variation in the sequence of the human genome, it is important to keep in mind that the 0.1 percent diff erence represents potential for variation in about three million nucleotides. Some of this variation is associated with many diseases. Scientists believe that almost all human diseases have a genetic component, either directly or indirectly. Comparing genome sequences has been particularly useful in studying the genetic basis for many human diseases, such as cancer. For example, bioinformatics and computational biology have been used to compare the DNA sequences of certain regions of the genome in individuals aff ected by a particular type of cancer with the DNA sequences of the same regions in those who are not. Diff erences in DNA sequence indicate a potential genetic basis for the disease. While this represents a good starting point for the study of the genetics of a disease, scientists are discovering that many diseases are the result of a complex array of factors, and studying them requires more elaborate methods.

genomics the study of genomes and the complex interactions of genes that result in phenotypes


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A

A

A

A

A

A

A

A

C

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SNPSNPSNP

Haplotype map of the human genome

Linking Genetic Variations to DiseaseIn previous sections, you learned about diseases that are associated with a mutation or mutations in a single gene, such as sickle cell anemia. Many other diseases, such as cancer, stroke, heart disease, diabetes, and asthma, are infl uenced by a combination of environmental and genetic factors.

Many scientists consider determining what variations in DNA sequence contribute to diff erent diseases to be one of the best opportunities to understand the complex causes of many human diseases. Th e most common type of variation between people is diff erences in individual nucleotides, as shown in Figure 6.21. For example, one person may have a C at a certain location, while another person may have a T. Th is type of genetic variation is called a single nucleotide polymorphism, or SNP (pronounced “snip”). A SNP can act as a marker for a gene or be associated with a gene if it is genetically linked to it. Recall that sequences of DNA are genetically linked when they are physically close to each other on a chromosome and tend to be inherited together. For example, if a SNP is common among people with high blood pressure, that could provide a marker for the location of a gene that is involved in the disease. However, there are almost 10 million diff erent SNPs that commonly occur in the human genome. Testing all of these is not feasible. Nevertheless, SNPs that are near each other on a chromosome tend to be inherited together. Th ese regions of genetically linked variations are called haplotypes. Certain tag SNPs can uniquely identify these haplotypes. Since there are far fewer of these types of SNPs, they can be used as a basis for comparing genetic variations and identifying genes that infl uence the health of an individual.

In 2002, an international group of researchers from Canada, the United States, Japan, China, Nigeria, and the United Kingdom collectively began the International HapMap Project. Th e major aim of this project is to develop a haplotype map (HapMap) of the human genome, which represents a map of the variations in the human genome. Th is can then be used by other scientists to identify the genetic basis for many human diseases.

Beyond the Genome SequenceAnalysis of the data generated from the Human Genome Project will continue for many decades. A signifi cant part of that research involves more than working at the level of the DNA sequence. For example, the fi eld of proteomics began when scientists recognized how important it is to understand the products of our genes—proteins. Based on the term genome, the term proteome was developed to refer to all of the proteins in an organism. Research studies in proteomics focus on studying the three-dimensional shape of proteins and eventually determining the functions of all the cellular proteins.

Figure 6.21 A haplotype map is constructed by identifying single nucleotide polymorphisms (SNPs) among a number of individuals.


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Studying Gene ExpressionToday, many scientists are studying what regulates the expression of genes. Th at is, they look at what infl uences whether a particular protein is produced from a certain gene and, if so, how much of the protein is made. An individual’s phenotype is the result of which genes are active—are being expressed—and which genes are inactive, or not being expressed. While all cells of an individual have the identical genetic material, the same genes are not expressed in the same way in every type of cell. For example, diff erences in gene activity can exist between healthy cells and cells of diseased tissue, such as cells of cancerous tumours.

Scientists now realize that some factors that aff ect gene expression can be inherited, but they are not due to changes in DNA sequence. Epigenetics is the study of how changes in the inheritance of certain traits or phenotypes are based on changes to gene function and not to changes in DNA sequence. Epigenetics diff ers from evolution because there is no change to the DNA sequence of a gene and epigenetic changes are not necessarily permanent. Epigenetic changes represent a response to an environmental condition that may be reversed once that condition changes, or soon aft er the change. Th e term epigenome refers to cellular material that is not part of the genome but that infl uences whether a gene is “turned on” or “turned off .” Identifying epigenetic factors is believed to be a next major frontier in biological sciences.

Studying Gene Expression Using MicroarraysA very important method that is used to study diff erences in gene activity is DNA microarray technology. In this technique, DNA is placed as spots on a glass plate, called a microarray plate. One slide can contain thousands of spots of DNA that correspond to certain parts of a genome, and that contain diff erent genes. Figure 6.22 shows an example of a microarray plate. Th is technique allows scientists to study the activity of up to thousands of genes at a time, under particular conditions. Studying the activity of so many genes at once tells scientists which genes are active or inactive under certain conditions, and gives them information on how this activity is co-ordinated among diff erent genes.

Figure 6.22 DNA microarrays allow scientists to see the activity of genes under certain conditions. The colour of each circle on a microarray plate like this one corresponds to the activity of a gene in the DNA spot on the plate.


S63_BIO11.indd 265 14/05/10 1:45 PM

Genetic Information: Public Benefits and Concerns Some of the most important benefi ts of genetic research are in the area of human medicine. Figure 6.23 illustrates this link between genetics and medical treatment. Studying the human genome as a whole may make it possible to develop drugs that are tailored to the expression of the genes associated with particular disorders, and to the unique genome of a patient.

In the future, researchers hope to use established links between genetic variation and risk of disease to provide better medical advice to patients. If the cost of DNA sequencing continues to decrease, individuals may have access to their genetic profi le—their complete genotype, including all of the various mutations linked to disease. Currently, doctors are only able to make generalized risk assessments based on medical history. Armed with a genetic profi le, however, genetic counsellors and doctors will be able to provide more specifi c risk assessments, design individualized prevention plans, and design genetically precise treatment programs.

What Can Happen to Information from a Genetic Profi le?Establishing genetic profi les for individuals, and making these profi les available to health-care providers, also creates ethical concerns. For example, • Could insurance companies deny coverage to people who have a genetic

predisposition for a particular disease?• Could potential employers have access to an individual’s genetic profi le and use it in

assessing whether to hire the person?• Should researchers be allowed to use the genetic profi les of individuals to help them

better understand the link between genome and phenotype?

Th e central issue in all of these ethical questions is who should have access to the information in a genetic profi le.

Ownership of Genetic InformationAll the data gathered through the Human Genome Project is publicly available. Having access to the data made it possible for scientists to share what they learned about human genetics. In other areas of genetics research, however, the relationship between public and private information is more complex.

In 2005, the National Geographic Society and the IBM company jointly launched the Genographic Project. Th is project uses DNA samples provided by hundreds of thousands of volunteers around the world to learn more about the migrations of ancient peoples. Using high-tech genetics tools and computer facilities, DNA sequences of the individuals are analyzed to better understand human genetic roots and how we all “connect” at the level of our DNA.

Studies such as the Genographic Project can contribute valuable information to researchers in many fi elds. But who owns the genetic information? For example, should companies have the right to sell DNA information to other companies without the permission of the people who provided the samples? Should companies that use DNA in medical research be required to share the results of their work with the individuals or communities whose genetic information was used?

Some people argue that genetic information is a natural resource that belongs to everyone. Others believe that genetic information about a person belongs only to that person. In addition, many think that if companies cannot earn a profi t from their research, there is little incentive for them to invest in genetic studies. In the world of genetics research, where is the boundary between public and private property?

genetic profile the complete genotype of an individual, including various mutations

Figure 6.23 This altered representation of the caduceus—a common symbol for medical practice—illustrates the link between genetics and treatment of disease.


S63_BIO11.indd 266 14/05/10 1:45 PM


Review Questions 1. K/U Th e Human Genome Project involved

sequencing the genomes of other organisms as well as of humans. Provide two reasons for why this was done.

2. C In this section, the human genome was compared to a book. Illustrate how the parts of a book—pages, paragraphs, sentences, words, and letters—can be used to represent chromosomes, chromatids, genes, and nucleotides.

3. K/U Describe three things about the human genome that scientists learned from the Human Genome Project.

4. K/U Although the Human Genome Project is complete, research based on its fi ndings continues. Describe two areas of current research that developed from it.

5. A Th e Human Genome Project cost billions of dollars to complete. Do you think it was worth it? Provide reasons that support your opinion.

6. T/I Th e two pictures below show scientists conducting genetic research in labs. Th e photo on the left was taken in the 1980s, and the photo on the right was taken in the 2000s. Describe how these photos refl ect the changes in genetic research that took place over this time period.

7. T/I Describe why you think the fi eld of bioinformatics was given that name. Provide a suitable alternative name for this fi eld of science.

8. K/U What is genomics? Describe the type of research that is involved and how it may help society.

9. K/U What is the HapMap project? What is its main goal?

10. T/I Explain how epigenetics suggests that the traits we inherit may not be due only to the DNA we receive.

11. K/U Describe how DNA microarray technology is used to study gene expression.

12. C Determining a genetic profi le can have its benefi ts and its risks. Use a table to list as many benefi ts and risks as you can.

13. C Should people be encouraged to have their genetic profi les determined, since this might prevent them from developing certain illnesses? Justify your answer using examples.

14. A Imagine that you have been hired by an international organization that establishes practices for scientists to follow when doing genetics research. Your job is to develop a policy on the collection and ownership of genetic information.

a. What are some of the issues you should consider? b. Based on the issues you listed, decide where you

stand on those issues and develop a policy that refl ects that stance.

c. Briefl y summarize how your policy will balance public and private interests.

15. C Write a paragraph expressing your opinion on whether employers should have to provide a work environment that suits a person’s genetic profi le.

Section Summary• Th e complete DNA sequence of the human genome was

determined as part of the Human Genome Project.• Th e fi eld of bioinformatics arose from the need to share

and maintain the large quantities of data collected from genomic research. It also provides tools for analyzing genomic data.

• Th ere is still much to be learned from the data generated from the Human Genome Project, particularly in identifying genes that are associated with human health.

• Current and future research in genomics may allow scientists to tailor preventative and curative treatments for individual patients based on their specifi c genetic profi les. However, ethical questions about who owns an individual’s genetic information continue to be debated.


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S k i l l C h e c k✓ Initiating and Planning

✓ Performing and Recording

✓ Analyzing and Interpreting

✓ Communicating

Safety Precautions

• Wash your hands when you have completed this investigation

Suggested Materials• seeds (Brassica rapa, radish,

or bean) • labels• paper towels• water• shoe boxes• petri dishes• graduated cylinder• light source

Environmental Infl uences on the Production of ChlorophyllChlorophyll is the molecule that allows plants to capture light energy from the Sun and use the energy to produce food in the form of sugars. Chlorophyll is also the pigment that gives leaves their green colour. Plants that produce chlorophyll appear green. If the production of chlorophyll is “turned off ,” the plant will become pale yellow, or even white. Th e production of chlorophyll is under genetic control.

Working in groups and using the materials provided, you will design and conduct an investigation to test the infl uence of light on the production of chlorophyll. Your investigation must enable you to draw conclusions about each of the following. • What is the minimum duration of exposure to light required to turn on the

production of chlorophyll?• Is the triggering event reversible? Th at is, does chlorophyll production start

and stop as environmental conditions change?

Plan Your Own I N V E S T I G AT I O N 6-A

When chlorophyll is no longer present, a green plant will become pale yellow or even white. This is similar to what happens to many trees during the autumn. In the spring and summer, tree leaves appear green because chlorophyll is being produced. With the change in environmental conditions that accompanies autumn, chlorophyll is no longer produced and other pigments in the tree leaves become visible. This results in the yellow, orange, and red “fall colours” of some trees.

Go to Organizing Data in a Table in Appendix A for help with designing a table for data.

Go to Constructing Graphs in Appendix A for information about making graphs.


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Pre-Lab Questions 1. Describe the genotype of the organisms you should use

that will allow you to test the eff ect of the environment on phenotype.

2. What is the diff erence between qualitative and quantitative data?

3. Diff erentiate among independent, dependent, and controlled variables.

QuestionHow does light infl uence the production of chlorophyll in germinating plants?

HypothesisFormulate a hypothesis to explain how light infl uences the activity of the genes responsible for chlorophyll production. Use this hypothesis as the basis of your experimental design.

Plan and Conduct 1. With your group, brainstorm several methods you

could use to test your hypothesis given the materials provided. As a group, select one method for your experimental design.

2. Identify the independent, dependent, and controlled variables, and the type of data you will collect.

3. As you prepare your procedure, be sure to consider the time required for each step.

4. Prepare the data table you will use to record your observations. Decide what form (such as the type of graph) you will use to present your results.

5. Review your procedure with your teacher. Do not begin doing the investigation until your teacher has approved your group’s procedure.

6. Record your observations in your table. Make notes about any fi ndings that do not fi t in your data table. Record any questions that come up as you conduct your investigation.

Analyze and Interpret 1. Did your observations support or refute your

hypothesis? Explain. 2. Did your investigation allow you to draw conclusions

about the inheritance of the genes that are involved in the production of chlorophyll? Why or why not?

3. Identify the variables you considered when designing your investigation. Explain why you needed to consider each variable to obtain scientifi cally valid results.

Conclude and Communicate 4. State your conclusions about the relationship between

exposure to light and the activity of the genes that are involved in the production of chlorophyll.

Extend Further

5. INQUIRY Could a diff erent hypothesis be consistent with the results of your investigation? How could you design an investigation to test this diff erent hypothesis?

6. RESEARCH What social benefi t could come from understanding the eff ect of light on chlorophyll production?


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A B

S k i l l C h e c k

Initiating and Planning

✓ Performing and Recording

✓ Analyzing and Interpreting

✓ Communicating

Materials• data on crosses

Sex-linked CrossesTh omas Morgan used Drosophila melanogaster, the common fruit fl y, extensively in his studies of sex-linked traits. In this investigation, you will model Morgan’s experiments using Drosophila melanogaster and use your results to confi rm sex-linked inheritance for the trait you chose to study.

Pre-Lab Questions1. How is a sex-linked recessive trait distinguished from an autosomal

recessive trait? 2. Describe the genotype of the P generation that could be used to model

Morgan’s studies of sex-linked genes in Drosophila. 3. What phenotype is expected in the F1 generation produced from the cross

described in question 2?

QuestionHow are sex-linked traits inherited in Drosophila melanogaster? How do actual results compare with theoretical ratios?

Organize the Data1. Choose one trait from the table below (eye colour, eye shape, or body

colour) to investigate.

Common Sex-linked Traits in Drosophila melanogaster

Trait Phenotype 1 Phenotype 2

Eye colour White Red

Eye shape Round Bar

Body colour Black Yellow

ThoughtLabI N V E S T I G AT I O N 6-B

Two forms of eye colour in fruit flies are white and red (A). Eye shape can be round (A) or appear as narrow bars (B).

Go to Organizing Data in a Table in Appendix A for help with designing a table for data.


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A B

Part I: The F1 Generation

2. Determine the genotype of the fl ies use for the P generation.

3. Construct a table to record your results. 4. Use a computer simulation program or obtain results

for the F1 generation from your teacher. Record the results in your table.

5. Before beginning Part II, complete the Analysis section of the investigation for Part I.

Part II: The F2 Generation

6. Determine the genotype of the fl ies for the F1 cross. 7. Construct a table to record your results. 8. Use a computer simulation program or obtain results

for the F2 generation from your teacher. Record the results in your table.

Analyze and InterpretPart I

1. From the data you recorded on the appearance of the fl ies in the F1 generation, which trait is dominant? Explain your answer.

2. Given your response to question 1, form a hypothesis about the phenotypic ratio that you will observe in the F2 generation.

Part II

3. Calculate an actual phenotypic ratio of the F2 generation from your results.

Conclude and Communicate 4. Describe the inheritance pattern for the trait you

studied in this investigation. 5. How does the actual phenotypic ratio you obtained

compare to the theoretical phenotypic ratio? Account for any diff erences.

Extend Further

6. INQUIRY In this investigation, you tracked the inheritance pattern of one sex-linked trait. Design an investigation that would track the inheritance of one of these traits and the autosomal trait of normal versus vestigial wings. Describe the results you expect.

7. RESEARCH Drosophila melanogaster has been used extensively in genetics research. What other traits have been studied in Drosophila? On which chromosomes are the genes for these traits located?

Two forms of body colour in fruit flies are black (A) and yellow (B).


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SUMMARYChapter 6

Beyond Mendel’s Observations of InheritanceSection 6.1

Some patterns of inheritance are more complex than those fi rst proposed by Mendel. These include codominant and incomplete dominant inheritance patterns. In addition, for some traits multiple alleles for a gene can exist within the population.

KEY TERMScodominancecontinuous variationheterozygous advantage

incomplete dominancepolygenic trait

KEY CONCEPTS• Incomplete dominance leads to the expression of an

intermediate phenotype. In the case of codominance, both alleles are fully expressed.

• Although an individual has only two alleles for any gene, multiple alleles for a gene may exist within the population.

• Environmental conditions can infl uence the expression of certain traits.

• Polygenic traits are controlled by more than one gene and can usually be identifi ed by continuous variation in phenotype.

Inheritance of Linked GenesSection 6.2

Some traits are inherited together, due to linked genes. Gene linkage includes sex-linked genes, which are on the sex chromosomes.

KEY TERMS

linked genes sex-linked trait

KEY CONCEPTS• Alleles of diff erent genes that are on the same chromosome

do not assort independently. These genes are said to be linked and their associated traits tend to be inherited together.

• Sex-linked traits are expressed in diff erent ratios by male and female off spring because they are determined by the segregation of X and Y chromosomes.

• Although sex-linked genes are linked to the X and Y chromosomes, Punnett squares can be used to predict genotypes and phenotypes.

The Future of Genetics ResearchSection 6.3

Current and future research in genetics involves studying how phenotypes result from complex interactions between genes and gene products.

KEY TERMSbioinformaticsgenetic profi le

genomics

KEY CONCEPTS• The complete DNA sequence of the human genome was

determined as part of the Human Genome Project.

• The fi eld of bioinformatics arose from the need to share and maintain the large quantities of data collected from genomic research. It also provides tools for analyzing genomic data.

• There is still much to be learned from the data generated from the Human Genome Project, particularly in identifying genes that are associated with human health.

• Current and future research in genomics may allow scientists to tailor preventative and curative treatments for individual patients based on their specifi c genetic profi les. However, ethical questions about who owns an individual’s genetic information continue to be debated.


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REVIEWChapter 6

Knowledge and UnderstandingSelect the letter of the best answer below. 1. Th e seed colour of a particular species of plant

is inherited through incomplete dominance. If a true-breeding plant with blue seeds is crossed with a true-breeding plant with yellow seeds, what is the expected seed colour of the off spring?

a. yellow b. green c. blue d. yellow and blue spots e. You cannot predict seed colour from the

information given. 2. Roan cows are the result of a codominant inheritance

pattern. In roan cows, the allele for white hair and the allele for red hair are both expressed. Which of the following is the most appropriate representation for codominant alleles?

a. Let W = allele for white hair, and let R = allele for red hair.

b. Let W = allele for white hair, and let r = allele for red hair.

c. Let w = allele for white hair, and let R = allele for red hair.

d. Let CW= allele for white hair, and let CR

= allele for red hair.

e. Let Cw= allele for white hair, and let CR

= allele for red hair.

3. A man with blood type O and a woman with blood type AB have a child. Which of the following are possible blood type(s) for the child?

a. O only b. AB only c. A or B d. A, B, or O e. A, B, AB, or O

4. Skin colour in humans ranges from very dark to very light. Which of the following most likely describes how skin colour is inherited?

a. principle of dominance b. incomplete dominance c. codominance d. polygenic inheritance e. environmental infl uence

5. Th e following pedigree follows the inheritance pattern of sickle cell anemia in a family. What is the sex, genotype, and phenotype of individual II-5?

I

II

III

21

5

4 531 2

21 3 4

a. unaff ected female, HbAHbS b. aff ected female, HbAHbS c. unaff ected male, HbSHbS d. aff ected male, HbSHbS e. unaff ected male, HbAHbA

6. An X-linked dominant allele is inherited from a heterozygous female by

a. all of her sons b. half of her sons c. all of her daughters d. none of her daughters e. all of her children

7. Which of the following most accurately describes the fi eld of genomics?

a. the study of haplotypes b. the study of how DNA is copied c. the study of how genes interact to produce a

phenotype d. the study of how genomes are formed e. the study of the inheritance pattern of genes

8. How has DNA microarray technology revolutionized the study of gene activity?

a. Gene expression in cells can now be studied. b. Th e proteins produced by genes have been

discovered. c. Many genes can be studied at the same time. d. Th e human genome has been completely sequenced. e. All of the proteins produced in a cell can now be

studied.


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REVIEWChapter 6

Answer the questions below. 9. A plant that produces white fl owers is crossed with a

plant that produces red fl owers. Describe the pattern of inheritance if the fl owers produced are

a. pink b. red and white spotted c. all red

10. What is the predicted phenotypic ratio in the F2 generation if two alleles are inherited by incomplete dominance?

11. What is heterozygous advantage? Provide an example. 12. Describe how multiple alleles infl uence inheritance of

a trait. Provide an example. 13. Height is an example of a polygenic trait. What aspect

of height suggests this? 14. What are linked genes? Explain why their inheritance

is not according to the law of independent assortment. 15. Parents who do not have symptoms of Duchenne

muscular dystrophy have a son with Duchenne muscular dystrophy. Which parent has passed the disease to their son? Explain your answer.

16. What is a person’s genetic profi le? What are some ethical issues concerning access to this information?

Thinking and Investigation 17. A man with straight hair has two children with a

woman who has curly hair. One child has straight hair, and one has wavy hair. What pattern of inheritance for hair type does this suggest?

18. Use the pedigree below to answer the following questions. Th e letters in the symbols indicate the blood type of each individual.

a. Determine the blood types of individuals I-4 and I-6.

b. Individual III-2 and a man with blood type AB have four children. Will any of these children have blood type O? Explain.

19. In foxes, a pair of alleles, CP and Cp, interact as follows: • CPCP is lethal, usually during an embryonic stage • CPCp produces platinum-coloured fur • CpCp produces silver foxes. Could a fox breeder establish a true-breeding variety

of platinum foxes? Explain. 20. A man with type B blood and a woman with type

AB blood have children. What blood types are possible among their children? What would tell you that the man is heterozygous for type B blood?

21. A woman with type AB blood has a child with the same blood type. What are the possible genotypes of the father?

22. What could be a genetic reason for the black area of fur forming aft er a cold pack has been placed on the back of this Himalayan rabbit?

23. Explain why genes that are far apart on a single chromosome may be inherited as though they are on diff erent chromosomes.

24. A horse breeder fi nds that one of his stallions has a genetic defect that aff ects the production of sperm. Th e gene associated with this trait is located on the Y chromosome. What is the possibility that the stallion’s female off spring could pass on this trait to their sons? Explain.

25. Fruit fl ies can have normal wings or stunted wings. In an investigation, you mate several normal-winged females with a male that has stunted wings. In the F1 generations, only the males have stunted wings. What can you conclude from this investigation?

26. Suppose that the fi rst dihybrid crosses Mendel performed had involved traits controlled by closely linked genes.

a. How would Mendel’s results have diff ered from the results he obtained for a dihybrid cross involving non-linked genes?

b. What hypothesis might Mendel have developed to explain his results?

c. What investigation could Mendel have conducted to test this hypothesis? What would he have observed?

I

II

III

A AB B ? O ?

OAABAB

O O O A B

2 3 41

43 6521

1 2 3 4 5

5 6

AB


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Communication 27. Rudy and Maria are expecting a baby. Th ey have

normal vision, but both of their fathers are colour vision defi cient (CVD). Th eir mothers have normal vision.

a. Draw a pedigree for their family. b. What is the probability that the baby will be a girl

with CVD? c. What is the probability that the baby will be a boy

with normal vision? 28. Th e closer genes are together on a chromosome, the

more likely they will assort together. Illustrate this concept with a model or diagram.

29. Variability and diversity of living organisms result from the distribution of genetic

materials during the process of meiosis. Mendel proposed the idea that all genes assort independently, producing off spring with a variety of traits whose distribution can be predicted mathematically. William Bateson and Reginald Punnett found that not all genes do assort independently. Develop a diagram that shows independent assortment and how linked genes contradict this theory.

30. Genetic and genomic research can have social and environmental implications.

Identify a potential scientifi c outcome of genomics research. Develop an illustration showing the possible social implications of achieving that outcome.

31. In this chapter, DNA sequences in a genome are compared to letters strung together in a book. Develop another analogy for DNA, chromosomes, genes, and nucleotides. Illustrate your analogy with a diagram or model.

32. Use a graphic organizer to summarize the uses of bioinformatics in genetics research.

33. Th ere are many benefi ts to genetics research, but there are also signifi cant ethical concerns. Use a concept map to illustrate some of the benefi ts and concerns that are associated with the diff erent genetics research topics discussed in this chapter.

34. Summarize your learning in this chapter using a graphic organizer. To help you, the Chapter 6 Summary lists the Key Terms and Key Concepts. Refer to Using Graphic Organizers in Appendix A to help you decide which graphic organizer to use.

Application 35. A farmer wants to breed a variety of taller corn.

a. How can the farmer use variation in the height of the current corn plants to produce taller corn plants?

b. Will the farmer’s work be most eff ective if height in corn plants is determined by polygenic inheritance, multiple alleles, or codominant alleles? Explain.

c. Th e farmer fi nds that many of the tallest corn plants are also very susceptible to a particular disease. How could the farmer design an investigation to fi nd out if the genes for height are linked to the genes that cause susceptibility to the disease?

d. If these genes are linked, what steps could the farmer take to create a breed of corn that is taller and more disease-resistant than the current corn crop?

36. Figure 6.17 provides a summary of some important discoveries in genetics research, including the Human Genome Project.

a. Research one development or discovery that is in the fi gure, including an aspect of the Human Genome Project. Choose a subject that you have not learned about in this unit.

b. As part of your research, fi nd out about at least one individual who is associated with the discovery or invention.

c. Summarize your fi ndings and develop a presentation that you could present to the class or another general audience. Make sure your presentation includes a discussion on the importance of the discovery in terms of its contribution to scientifi c research.

37. Genome Canada was established in 2000 to develop a national program for fi nancial support of genomic and proteomic research in Canada.

a. Choose a research project that is funded by Genome Canada and that is listed on the Genome Canada website.

b. Write a brief description that summarizes what the project is studying. Include the names of the individuals associated with the project and at what institution(s) they work.

c. Research Genome Canada’s GE3LS program. What does this acronym stand for and what are the main objectives of this program? Develop an argument for or against the importance of having such a program.


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Chapter 6 SELF-ASSESSMENT

Select the letter of the best answer below. 1. K/U Incomplete dominance occurs when

a. one allele masks the expression of the other allele b. one trait is masked by the presence of another trait c. both alleles are expressed when the alleles occur

together d. an intermediate phenotype is expressed when the

alleles occur together e. an unpredictable phenotype is expressed when the

alleles occur together 2. K/U Which of the following is an example of

codominance? a. A plant with green seeds is crossed with a plant with

white seeds; the off spring produce white seeds. b. Individuals who are heterozygous for sickle cell

disease produce both normal and sickle-shaped red blood cells.

c. A red snapdragon crossed with a white snapdragon produces pink snapdragons.

d. Th ere are many genes that control eye colour. e. A litter of kittens oft en display a wide variety of

traits. 3. T/I A man who is homozygous for blood type A

and a woman who is homozygous for blood type B have a child. Which of the following could be the child’s genotype?

a. IAi b. IAIA c. IBi d. IBIB e. IAIB

4. K/U Which two terms are most relevant to the inheritance of human blood types?

a. incomplete dominance and codominance b. codominance and multiple alleles c. incomplete dominance and multiple alleles d. codominance and polygenic inheritance e. dominance and codominance

5. K/U Traits that exhibit continuous variation are usually

a. controlled by one gene b. the result of codominance c. dominant d. polygenic e. aff ected by the environment

Use the following information to answer questions 6 and 7.Th e gene that controls coat colour in rabbits has four alleles: agouti (C), chinchilla (cch), Himalayan (ch), and albino (c). Th e order of dominance is C > cch > ch > c. 6. K/U What is the phenotype of a rabbit with the

genotype cchc? a. agouti b. chinchilla c. chinchilla and albino mix d. Himalayan e. albino

7. T/I If a rabbit with the phenotype cchch is crossed with an albino rabbit, what is the probability of producing a Himalayan rabbit?

a. 0 percent b. 25 percent c. 50 percent d. 75 percent e. 100 percent

8. K/U How can linked genes become “unlinked”? a. During meiosis, they sort independently. b. During crossing over, they are separated. c. During anaphase, they segregate to opposite poles

in the cell. d. During mutation, the genes are separated. e. During DNA replication, the genes are rearranged.

9. T/I Hemophilia is an X-linked recessive disorder. If a female with hemophilia and a male without hemophilia had children, what is the predicted percentage of children who would have hemophilia?

a. 0 percent b. 25 percent c. 50 percent d. 75 percent e. 100 percent

10. K/U Which of the following statements about the Human Genome Project is false?

a. It involved sequencing the human genome. b. It identifi ed coding and non-coding sections of

DNA. c. It involved sequencing the genome of common

representative organisms. d. It identifi ed genes in the human genome. e. It determined the functions of the genes in the

human genome.


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Self-Check

If you missed question... 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Review section(s)... 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.2 6.2 6.3 6.1 6.1 6.1 6.1 6.2 6.2 6.2 6.2 6.2 6.2 6.3 6.3 6.3 6.3 6.3

Use sentences and diagrams as appropriate to answer the questions below.11. T/I In radishes, colour is controlled by two alleles,

one for red colour and one for white colour. Inheritance of these alleles shows incomplete dominance. Th e photographs below show the phenotype for each possible colour: red, purple, and white. What phenotypic ratio would you expect from crossing two heterozygous radish plants?

12. T/I A student crosses a true-breeding plant that produces green seeds with a true-breeding plant that produces yellow seeds. Predict the possible off spring whena. the allele for green seeds is dominant to the allele

for yellow seedsb. the allele for green seeds is codominant with the

allele for yellow seedsc. the alleles for green and yellow seeds are

incompletely dominant 13. C Blood type ABO is determined by three alleles.

Draw a diagram that shows how blood type is determined by a combination of the three alleles.

14. A Investigating environmental eff ects on gene expression is an important aspect of genetics research on plant crops. Explain why, using an example of a trait to illustrate your answer.

15. C Draw a diagram that illustrates the concept of linked alleles of genes. In your diagram, show how they can become unlinked.

16. C A female fruit fl y that is homozygous dominant for red eyes is crossed with a white-eyed male fruit fl y. Use a Punnett square to predict the genotype(s) and phenotype(s) of their off spring.

17. T/I Th e pedigree below illustrates the sex-linked inheritance pattern of a trait in a family.

I

II

III

6

5 6

54

3 4

32

1 2

1

21

a. Explain how this pedigree shows sex-linked inheritance. What type of sex-linked inheritance is it? Explain.

b. From the pattern of inheritance you determined in part (a), determine the genotype of II-2.

c. Based on your answer to part (a), determine the probability that individuals II-1 and II-2 would have an aff ected child.

18. C Duchenne muscular dystrophy aff ects many more males than females. Explain why and draw a pedigree to illustrate its inheritance pattern.

19. K/U Explain why males cannot be carriers of an X-linked trait.

20. K/U Explain how Barr bodies account for the patchy colours of female calico cats.

21. K/U Why did the Human Genome Project include the sequencing of other organisms?

22. C “Decoding the human genome can be compared to reading a book in a language that nobody knows or understands.” Explain this statement using diagrams or a graphic organizer.

23. A What is genomics research? How can it be used to improve human health?

24. A What is bioinformatics? Describe a scientifi c study that uses bioinformatics.

25. A Th e human genome has long stretches of DNA that do not code for proteins. Describe how the variation between individuals in these regions can be useful to study.


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Documents

CHAPTER 6 Complex Patterns of Inheritance