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BIOLOGY 3201 – Unit 3A Mendelian Genetics
Name: ____________________ Slot: ______
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BIOLOGY 3201 – Unit 3A
Mendelian Genetics
Introduction to Genetics
For centuries, people have known that certain physical characteristics are passed from one generation to the next. Using this knowledge, they learned to produce crops and livestock with desired characteristics. However, how these characteristics are passed from one generation to the next was unknown to them.
Traits - Distinguishing or unique characteristics which make one organism different from other organisms.
n Some traits are desirable while others are not n Can you think of any undesirable traits? Desirable?
It can be observed that traits can be passed down from one generation to the next (i.e. parents to offspring). This transmission of traits is called heredity and the traits which are passed on are said to be inherited.
What is GENETICS?
Genetics is a branch of Biology which is concerned with studying the inheritance of traits and the variations caused by them.
By studying genetics we gain a better understanding of how we can determine the inheritance of certain traits and patterns involved in their inheritance.
The knowledge of genetics which we have today is a far cry from what we knew in the past.
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Past Genetics
Hippocrates (460 - 377 BC), a Greek philosopher, theorized that every part of the body was involved in the production of the “seeds” which the parent produced. The seeds of the male and female parent fused together to produce a new individual.
In the 18th century, scientists believed that sperm contained pre-formed embryos. Thus it was the male who had a major contribution to the new individual which was being produced. The contribution of the female was small.
In 1853, a monk named Gregor Mendel performed a number of experiments which involved pea plants. This study took place over an eight year period and the results of these experiments laid down a basis of inheritance from which other studies were done. Mendel is known as the ‘Father of Genetics’.
Mendel - Why Pea Plants?
Mendel chose the pea plants because:
1. Pea plants were commercially available throughout Europe at this time
2. Pea plants are easy to grow and mature quickly
3. The structure of the pea plants reproductive organs allowed Mendel control which plants reproduced
4. He cross-pollinated and self-pollinated these plants
5. Different varieties of the pea plant had different traits which could be observed easily from one generation to the next
Mendel’s Experiments
Mendel examined seven different traits in pea plants
n Each trait had only two possible forms or variations.
n In order to perform his experiments, Mendel bred his pea plants until he obtained purebred plants.
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n A purebred organism is similar to the parent or parents which produced it.
n These purebred plants were true breeding plants which produced plants with the desired features that Mendel was trying to obtain.
n For example, a tall parent plant would only produce tall offspring plants.
Purebred = homozygous Hybrid = heterozygous
Mendel’s First Experiment Monohybrid Cross (one factor)
Once he obtained purebred (homozygous) plants for each of the traits which he was using, he called these the parent or P generation.
He crossed these parent plants to obtain a first generation of offspring which he called the first filial generation or F1 generation.
The plants which were produced in the F1 generation were called hybrids (heterozygous) because they were the result of a cross between two different purebred plants.
When two plants from the F1 generation were crossed, the offspring were called the second filial generation or F2 generation
Since only one trait was being considered in these crosses, they are called monohybrid crosses
Monohybrid Cross
n When Mendel performed his cross for the trait of plant height, he crossed a purebred tall plant with a purebred short plant.
n Mendel expected the offspring to be medium height. What height would you expect the offspring plants to be?
n This was not the case, all the offspring were tall.
n From this observation he concluded that the trait for tall was dominant and the trait for short was recessive.
n Both forms of the trait were present in the F1 plants, but the short form could not be seen since it was being dominated by the tall form.
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n A dominant trait is a characteristic which is always expressed or always appears in an individual (use a capital letter) ‘G’
n A recessive trait is a characteristic which is latent or inactive and usually does not appear in an individual (use a lowercase letter) ‘g’
n From this Mendel formed what he called the Principle of Dominance
Principle of Dominance
When individuals with contrasting traits are crossed, the offspring will express only the dominant trait.
Dominant forms of a trait will always be expressed when it is inherited. Found in hybrids (heterozygous individuals) Found in pure dominant (homozygous dominant)
§ GG = Yellow § Gg = Still Yellow § gg = green (recessive)
What did this tell him about yellow vs. green color?
Mendel’s Second Experiment Crossing the F1 generation
n He took TWO of the plants produced during his first test (F1 Generation), and bred them together to make another generation of offspring (F2 Generation).
n When two of these F1 hybrids (dominant looking heterozygous plants) are crossed, the result is an F2 generation showing 3 dominant for every 1 recessive.
n When Mendel crossed two F1 offspring to obtain the F2 offspring he obtained the following results every time.
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THIS IS CALLED A:
“Mendelian Ratio” (expressing 75% dominant, 25% recessive, 3:1)
Mendelian Ratios: 3:1 phenotype ratio - dominant to recessive Phenotype (what you can PHysically see)
Genotype – the GEnetic makeup of the alleles. 1 homozygous dominant – 2 heterozygous – homozygous recessive
1:2:1 Genotype Ratio
Mendel’s Law of Segregation
Based on the repeated pattern of results after completing thousands of crosses, Mendel derived the 1st Law of Genetics. LAW OF SEGREGATION Each parent in any generation starts with two hereditary factors. (Either both dominant, both recessive, or a combination of dominant & recessive.) Only one factor from each parent is contributed to the offspring.
Segregation – Key Points
1) Each parent in the F1 generation started with two hereditary “factors”. One was dominant while the other was recessive
2) The factors separate in the parent. Only one factor from each parent is contributed to the offspring.
3) Each offspring inherits one factor from each parent. If the dominant factor is present it will be expressed even if the recessive factor is also present.
4) The recessive factor will be expressed only if another recessive factor is present as well. We know today that Mendel’s “factors” were genes.
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Genes - the part of a chromosome that governs the expression of a particular trait. Allele - alternate form of a gene (blue vs. brown eyes)
Two letters are necessary to describe each combination of alleles.
Example: T - tall t- short
TT- Tall Tt- Tall tt – short
n When two alleles are the same (both dominant or both recessive) we say the plant (or animal) is homozygous.
n When two alleles are different (one dominant and one recessive) we say the plant (or
animal) is heterozygous.
Unit Theory
The genes/factors for one trait are inherited as a unit ....the “Unit Theory of Inheritance”. Why Units?? Remember the inner workings of a cell, i.e. chromosomes/DNA have not been identified yet!
Punnett Squares
Devised by Reginald C. Punnett, a British mathematician. It is used to calculate the probability of inheriting a particular trait.
n All possible gametes of one parent are listed across the top and all the possible gametes for the other parent are listed down the side.
n When the square is filled in by copying the row and column-head letters across or down the empty squares the outcome for a cross for a given set of alleles is given.
In a Punnett square for a monohybrid cross, the possible gametes of the parents are put on the side of the table and the Principle of Segregation is applied.
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Let’s try a cross using two of the heterozygous plants from Mendel’s F1 generation
Always show the parents genotype first: (This will help to set up your Punnett Square)
P - ____ _____ x ____ _____ F2 Cross Phenotype: Genotype:
No matter what physical trait Mendel observed, (color, shape etc.) a 3:1 phenotypic ratio of characteristics was always found in the F2.
What Works for Peas Also Works for Humans
In the cross Aa x Aa, where A is a dominant allele for wild type (standard) pigmentation and a is a recessive allele for no pigmentation (albinism):
Legend: A – ________________ P - ____ _____ x ____ _____ a – ________________
¾ of offspring will be wild type and ¼ will be albino
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Try this One Factor Cross
In pea plants, round seeds are dominant to wrinkled seeds. If a homozygous round seed plant is crossed with a heterozygous round seed plant, what is the expected phenotypic and genotypic ratios in the F1?
R - _____________ P - ____ _____ x ____ _____ r - _____________
Results: **CHECK ANSWERS ON BOARD
Try this one!
Fruit fly wing length is controlled by a dominant allele for long wings (L), short is recessive. If a heterozygous long winged fly is mated to a homozygous short winged fly, what is the expected phenotypic and genotypic ratio in their offspring?
We will do lots of practice problems like this in the next few weeks J
Test Cross
To determine the UNKNOWN genotype of an organism that shows the dominant phenotype, we perform a TEST CROSS.
n This is done by taking the organism with the unknown genotype (TT or Tt) and cross it with a purebred (homozygous) recessive (tt) and look at the results.
n If any of the offspring show the recessive condition then the unknown parent MUST have been heterozygous.
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The Product Rule
A rule that uses the principles of probability to determine the possible outcomes of a genetic cross.
Product Rule: The probability, or chance, that two or more independent events will occur together is the product of their individual probabilities of occurring alone.
Example: Tossing coins
Toss a single coin: Chances of getting heads = ½ Chances of getting tails = ½
Toss two coins together: Chance of getting two heads = ½ x ½ = ¼ Chance of getting two tails = ½ x ½ = ¼
What is the probability that a couple’s first offspring will be a male
and their second offspring will be a female?
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Mendelian Genetics – Problems involving ONE gene
1. Perform the indicated crosses. Show all workings including Punnett squares.
In cows, black coat (B) is dominant to white coat (b). What are the expected phenotypic and genotypic ratios in the F1 generation for a cross between two cows that are:
A. Heterozygous for black crossed with heterozygous for black
B. Heterozygous for black crossed with homozygous for black
C. Homozygous for black crossed with homozygous for white
D. Heterozygous for black crossed with homozygous for white
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2. In peas, the gene for red colour dominates over the gene for green colour. Determine the expected ratios in the F1 generation for a cross between a red homozygous pea plant and a green homozygous pea plant. Show your workings.
3. In peas, the gene for wrinkled seeds dominates over the gene for smooth seeds. Determine each of the following:
A. The phenotypic and genotypic ratios of the F1 generation if two heterozygous plants are crossed
B. If 3470 seeds were produced from this cross, approximately how many would you expect to be smooth seeds?
C. How many seeds would you expect to have the heterozygous geneotype?
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For the following problems, get in the habit of writing down the information you are provided and any work necessary to solve the problems.
4. In cats, short hair is recessive to long hair. A true-breeding (homozygous) long-haired male is mated to a short-haired female. What will their kittens look like?
5. Two cats are mated. One of the parent cats is short-haired (recessive allele). The litter which results contains two long-haired and three short-haired kittens. What does the second parent look like, and what is its genotype?
6. Mrs. and Mr. Smith both have widow’s peaks (dominant). Their first child also has a widow’s peak, but their second child doesn’t. Mr. Smith accuses Mrs. Smith of being unfaithful to him. Is he necessarily justified? Why or why not? Work the genetics problem predicting the frequencies of the versions of this trait among their prospective children.
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BIKINI BOTTOM GENETICS
Scientists at Bikini Bottom have been investigating the genetic makeup of the organisms in this community. Use the information provided and your knowledge of genetics to answer each question.
1. For each genotype below, indicate whether it is a heterozygous (He) OR homozygous (Ho).
TT _____ Bb _____ DD _____ Ff _____ tt _____ dd _____
Dd _____ ff _____ Tt _____ bb _____ BB _____ FF _____
Which of the genotypes above would be considered purebred?
Which of the genotypes above would be hybrids?
2. Determine the phenotype for each genotype using the information provided about SpongeBob.
Yellow body color is dominant to blue
YY __________________ Yy __________________ yy __________________
Square shape is dominant to round
SS __________________ Ss __________________ ss __________________
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3. For each phenotype, give the genotypes that are possible for Patrick.
A tall head (T) is dominant to short (t)
Tall = __________________ Short = __________________
Pink body color (P) is dominant to yellow (p)
Pink body = __________________ Yellow body = __________________
4. SpongeBob SquarePants recently met SpongeSusie Roundpants at a dance. SpongeBob is heterozygous for his square shape, but SpongeSusie is round. Create a Punnett square to show the possibilities that would result if SpongeBob and SpongeSusie had children.
A. List the possible genotypes and phenotypes for their children
B. What are the chances of a child with a square shape? ____ out of ____ or ____%
C. What are the chances of a child with a round shape? ____ out of ____ or ____%
5. Patrick met Patti at the dance. Both of them are heterozygous for their pink body color, which is dominant over yellow body color. Create a Punnett square to show the possibilities that would result if Patrick and Patti had children.
A. List the possible genotypes and phenotypes for their children
B. What are the chances of a child with a pink body? ____ out of ____ or ____%
C. What are the chances of a child with a yellow body? ____ out of ____ or ____%
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6. Everyone in Squidward’s family has light blue skin, which is the dominant trait for body color in his hometown of Squid Valley. His family brags that they are a “purebred” line. He recently married a nice girl who has light green skin, which is a recessive trait. Create a Punnett square to show the possibilities that would result if Squidward and his new bride had children. Use B to represent the dominant gene and b to represent the recessive gene.
A. List the possible genotypes and phenotypes for their children.
B. What are the chances of a child with light blue skin? ____%
C. What are the chances of a child with light green skin? ____%
D. Would Squidward’s children still be considered purebreds? Explain!
7. Assume that one of Squidward’s sons, who is heterozygous for the light blue body color, married a girl that was also heterozygous. Create a Punnett square to show the possibilities that would result if they had children.
A. List the possible genotypes and phenotypes for their children.
B. What are the chances of a child with light blue skin? ____%
C. What are the chances of a child with light green skin? ____%
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8. Mr. Krabbs and his wife recently had a Lil’ Krabby, but it has not been a happy occasion for them. Mrs. Krabbs has been upset since she first saw her new baby who had short eyeballs. She claims that the hospital goofed and mixed up her baby with someone else’s baby. Mr. Krabbs is homozygous for his tall eyeballs, while his wife is heterozygous for her tall eyeballs. Some members of her family have short eyes, which is the recessive trait. Create a Punnett square using T for the dominant gene and t for the recessive one.
A. List the possible genotypes and phenotypes for their children.
B. Did the hospital make a mistake? Explain your answer.
9. Use the information for SpongeBob’s traits to write the phenotype (physical appearance) for each item.
(a) LL-____________ (e) Rr-_____________ (b) yy-_____________ (f) ll- _____________ (c) Ss-_____________ (g) ss- _____________ (d) RR-___________ (h) Yy-_____________
10. Use the information in the chart in #1 to write the genotype (or genotypes) for each trait below.
(a) Yellow body - _____________ (e) Stubby nose - _____________
(b) Roundpants - _____________ (f) Round eyes - ______________
(c) Oval eyes - _______________ (g) Squarepants - _____________
(d) Long nose - ______________ (h) Blue body - ______________
11. Determine the genotypes for each using the information in the chart in #1.
(a) Heterozygous round eyes -_____ (c) Homozygous long nose - ______
(b) Purebred squarepants - ______ (d) Hybrid yellow body - ______
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12. One of SpongeBob’s cousins, SpongeBillyBob, recently met a cute squarepants gal, SpongeGerdy, at a local dance and fell in love. Use your knowledge of genetics to answer the questions below.
(a) If SpongeGerdy’s father is a heterozygous squarepants and her mother is a roundpants, what is her genotype? Complete the first Punnett square to show the possible genotypes. Based on your results, what would Gerdy’s genotype have to be? ________
(b) Complete the second Punnett square to show the possibilities that would result if BillyBob & Gerdy had children. NOTE: SpongeBillyBob is heterozygous for his squarepants shape.
(c) What is the probability of kids with squarepants? _____ %
(d) What is the probability of kids with roundpants? _____ %
13. SpongeBob’s aunt and uncle, SpongeWilma and SpongeWilbur, have the biggest round eyes in the family. Wilma is believed to be heterozygous for her round eye shape, while Wilbur’s family brags that they are a pure line. Complete the Punnett square to show the possibilities that would result if SpongeWilma and SpongeWilbur had children.
(a) Give the genotype for each person. Wilma - _______ Wilbur - ________
(b) Complete the Punnett square to show the possibilities that would result if they had children.
(c) What is the probability that the kids would have round eyes? ____ %
(d) What is the probability that the kids would be oval eyes? ____ %
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14. SpongeBob’s mother is so proud of her son and his new wife, SpongeSusie, as they are expecting a little sponge. She knows that they have a 50% chance of having a little roundpants, but is also hoping the new arrival will be blue (a recessive trait) like SpongeSusie and many members of her family. If SpongeBob is heterozygous for his yellow body color, what are the chances that the baby sponge will be blue? Use the Punnett square to help you answer this question.
15. SpongeBob’s aunt is famous around town for her itty, bitty stubby nose! She recently met a cute squarepants fellow who also has a stubby nose, which is a recessive trait. Would it be possible for them to have a child with a regular long nose? Why or why not? Use the Punnett square to help you answer this question. 16. If SpongeBob’s aunt described in #7 wanted children with long noses, what type of fellow would she need to marry in order to give her the best chances?
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Mendel’s Second Experiment – A Dihybrid Cross
Dihybrid cross – cross of two heterozygous individuals for two different traits
n Mendel wanted to know if the inheritance of one characteristic influenced another (for example, does height affect the pea shape). So he crossed two plants that were purebred for two different traits.
n Mendel crossed plants that had round, yellow seeds with plants with green, wrinkled seeds (round and yellow are dominant traits). The F1 generation had seeds which were all yellow and round.
n Note: Notice the numbers for the phenotype and genotype of the F2: Anything similar?
Law of Independent Assortment
Phenotype: 9:3:3:1 Genotype: 1:1:2:2:4:2:2:1:1
These ratios were obtained repeatedly which helped Mendel in the statement of the Law of Independent Assortment LAW OF INDEPENDENT ASSORTMENT The genes/factors for one trait are inherited as a single unit and are inherited separately (independently) from other traits.
Dihybrid Cross
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Punnett Square – Dihybrid Cross (Two factors) Step by Step
#1 - Make a legend
#2 – Show parents 4 dashes times 4 dashes.
____ ____ ____ ____ x ____ ____ ____ ____
NOTE- Remember two dashes for each trait!!
#3 Determine all possible gamete combinations (should always be FOUR)
#4 Make your Punnett square.
Tip- Keep same letter together in the boxes and always put the capital letter first (when possible)
TRY THIS!
Tall is dominant over short. Yellow is dominant over green. Cross a homozygous tall, homozygous green plant with a plant that is heterozygous for both traits.
Legend:
P: ____ ____ ____ ____ x ____ ____ ____ ____ Results:
Genotypes: Phenotypes:
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Beyond Mendel’s Laws
Mendel’s experiments focused on traits that involved only two alleles where one factor was clearly dominant over the other. However, this is not always the case.
Dominance is not always complete
n Crosses between true-breeding strains can produce hybrids with phenotypes different from both parents
Ex.
n Incomplete Dominance n Co-Dominance
n Multiple Alleles
Incomplete Dominance
This occurs in the case where two pure breeding (homozygous) parents that show two completely different phenotypes have F1 hybrids that show a completely different phenotype from either parent.
n The F1 expresses an intermediate phenotype n Neither allele is dominant or recessive to the other n Phenotypic ratios are same as genotypic ratios
Incomplete Dominance for Flower Color in Snapdragon
Red snapdragons have two red alleles, white snapdragons have two white alleles, and pink snapdragons result from one red allele and one white allele.
Note that R stands for red and R’ stands for white since neither allele is dominant
n What would happen if TWO PINK were crossed? Try it!
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1. What are the expected results when a homozygous red flower is crossed with a pink flower? These
flowers show incomplete dominance. Show your workings.
2. What are the expected results when a homozygous black corn is crossed with a homozygous white
corn that shows incomplete dominance?
3. In corn, the gene for black color is incompletely dominant to the gene for white color. What are the
expected ratios for a cross between a corn that is beige and one that is homozygous black?
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4. A cross between a blue blahblah bird and a white blahblah bird produces offspring that are silver. The
color of blahblah birds is determined by just two alleles.
a) What are the genotypes of the parent blahblah birds in the original cross?
b) What is/are the genotype(s) of the silver offspring?
c) What would be the phenotypic ratios of offspring produced by two silver blahblah birds?
5. The color of fruit for plant "X" is determined by two alleles. When two plants with orange fruits are
crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit,
25% yellow fruit. What are the genotypes of the parent orange-fruited plants?
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6. Mr. and Mrs. Anderson both have tightly curled hair. (The hair form gene shows incomplete
dominance. There are two alleles, curly and straight. The heterozygote has wavy hair.) The Andersons
have a child with wavy hair. Mr. Anderson accuses Mrs. Anderson of being unfaithful to him. Is he
necessarily justified? Why or why not?
7. Two wavy haired people (one male and one female) marry and have eight children. Of these eight,
how many would you expect to be curly haired, how many wavy haired and how many straight haired?
Suppose you examine the actual children and discover that three of the eight have curly hair. What do
you suppose went wrong?
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Co-Dominance
Co-Dominance occurs when parents have different traits and the alleles for these traits are equally dominant to the other. The F1 hybrid offspring of a cross between two purebreds (homozygous) will express the phenotype of both parents equally.
n Phenotypic ratios are same as genotypic ratios n For example, feather color in chickens governed by two dominant alleles, black (B)
and white (W). Birds that are heterozygous (BW) have a striped pattern
Three types of DOMINANCE
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1. In cattle, the genotypes for red fur is co dominant with the gene for white fur. This results in cows
displaying Roan (red + white) fur color.
A. What are the genotypes and phenotypes of a cross between a red cow and a roan cow?
B. What should the genotypes and phenotypes for parent cattle be if a farmer wanted only cattle with
red fur?
2. A cross between a black cat and a tan cat produces a tabby pattern (black and tan fur together)
A. What pattern of inheritance does this illustrate?
B. What percent of kittens would have tan fur if a tabby cat is crossed with a black cat?
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Multiple Alleles
In many species a gene may have more than two possible alleles for any given gene.
n Ex. Human Blood Groups (A, B, and O) n (A & B are co-dominant and O is recessive)
Blood groups are determined by one gene with three alleles.
Also that the blood groups show both complete dominance and codominance.
Mixing Blood
We all know that only certain types of blood cannot be mixed with other types of blood. This is because if you mix the wrong types of blood agglutination or clumping will occur. This will causes the blood cells in the person being affected to clump together and the person would die from a blood clot in the brain or heart etc.
Q. What types of blood can be mixed together?
A. To answer this question we should look at the alleles for each blood type.
Blood Type/Allele Blood it can mix with - safely A – IAIA or IAi A or AB (Must have the A allele)
B – IBIB or IBi B or AB (Must have the B allele)
AB – IAIB AB only! (Must have A and B alleles)
O – ii Any of the blood types (Recessive, so it can mix with any blood type)
NOTE: AB is the universal ACCEPTOR (it can accept all blood types) O is the universal DONOR (can donate to all blood types because it is recessive)
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MULTIPLE ALLELES QUESTIONS
Blood type is controlled by three alleles, A, B, O. A and B are co-dominant, O is recessive.
1A. What are the two genotypes possible for a person who has A blood? _________________
B. What genotype does a person with AB blood have? _________________
C. What genotype does a person with O blood have? _________________
D. What are the two genotypes possible for a person who has B blood? _________________
2. Fill in the following chart:
Blood Type Antigen Present Anti-body Present Safe to Receive Unsafe to receive
A A B A, O B, AB
B
AB
O
A. According to the table, which blood type can be given to anyone? _______
B. According to the table, which blood type is able to accept all of the blood types? _______
3. A man with type AB blood is married to a woman also with type AB blood. What
percentage of their children will have?
A blood? _______ B blood? _______ O blood? _______ AB blood? _______
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4. A man has type B blood (genotype IBIB) is married to a woman with type O blood. What
blood type will all their children have? What is the genotype of their children?
5. A woman with type A blood (genotype IAi) is married to a type B person (genotype IBi). What
proportion of their children will have?
A blood? _______ B blood? _______ O blood? _______ AB blood? _______
6. A woman with type A blood is claiming that a man with type AB blood is the father of her child
who is type B. Could this man be the father of the child? Show the possible crosses; remember
that the woman can have IAIA or IAi genotypes. Assuming that he is the father, what must the
mother’s genotype be?
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7. A man with type AB blood is married to a woman with type O blood, They have two biological
children and one adopted child. Jane has type A blood, Bobby has type B blood and Grace has
type O blood. Which child was adopted?
8. A woman has a daughter. There are three men whom she claims might have been the father of
the child. The judge in the paternity court orders all three men, then child and the mother to have
blood tests. The results are: Mother: Type A, Daughter: Type O, Man #1: Type AB, Man #2: Type
B, Man #3: Type O. The mother claims that this proves that Man #3 must be the little girl’s father.
Is the mother correct?
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9. Mr. Metcalf, a hospital director calls you in a state of panic. When he has finally calmed down
enough to talk coherently, you learn that one of his interns came on duty the previous night after
spending a few hours with his friends at a local bar. As a joke, he decided to exchange wrist bands
on the four babies in the nursery, all of whom are the same age, have tiny wrinkly hands and are
indistinguishable to the families. As a first step in unraveling this mess you blood type the parents
and babies, with the exception of Mr. and Mrs. Blatz who refuse to do anything until they have
talked with their lawyer. The results of the parents’ blood tests are:
Mrs. Frank – AB, Mr. Frank – O
Mrs. Zeeb – B Mr. Zeeb – B
Mrs. Youngblood – O Mr. Youngblood – O
Mrs. and Mr. Blatz – unknown
Assume that the individuals listed above are the true parents – each set of parents having had one of the
four babies. Indicate the one family to which each of the following children belongs to on the basis of
blood types. Explain your reasoning.
Baby 1: B blood type
Baby 2: A blood type
Baby 3: AB blood type
Baby 4: O blood type
10. In a particular family, one parent has Type A blood, the other has Type B. They have four children.
One has Type A, one has Type B, one has Type AB and the last has Type O. What are the
genotypes of all six people in this family?
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Do variations on dominance relations negate Mendel’s law of segregation?
n Dominance relations only affect the relationship between genotype and phenotype n Alleles still segregate randomly and unite randomly n Gene products (plus external factors) control the expression of phenotypes
Pedigree Charts
Human experimental crosses are not possible in genetic studies. Therefore, human geneticists use medical, historical, and family records to study crosses that have already occurred. Records that have extended across several generations can be arranged in the form of a family pedigree. A pedigree is a diagram that illustrates the genetic relationship among a group of related individuals. Squares represent males and circles represent females. If the square or circle is coloured, it means that the individual is dominant or recessive for a single trait. If the circle or square is only half - coloured, it means that the individual is heterozygous or a carrier for that trait. Generations are indicated by Roman numerals to the left of the chart.
Pedigree – diagram that illustrates the genetic relationships among a group of individuals
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Chromosome Theory of Inheritance
When Mendel did his experiments with pea plants, he did not know that chromosomes existed in cells. In the early 1900s, chromosomes were
discovered and observed in cells.
In 1902, two scientists Walter Sutton and Theodor Boveri were studying meiosis and found that chromosomes behaved in a similar way to the factors (genes) which Mendel described.
Sutton and Boveri made three observations:
n Chromosomes occur in pairs and these pairs segregate (separate) during meiosis. n Chromosomes align independently of each other along the equator of the cell during
meiosis. n Each gamete (sex cell) receives only one chromosome from each pair.
From the above observations, they formed the Chromosome Theory of Inheritance. This theory states:
n Mendel’s factors (genes) are carried on chromosomes
The segregation and independent assortment of chromosomes during meiosis accounts for the pattern of inheritance in an organism
Thomas Morgan (Gene-Chromosome Theory)
In 1910, an American scientist called Thomas Morgan made a very important discovery from his work with fruit flies.
n Found that chromosomes in fruit flies are the same except for one pair.
n He called the dissimilar pair sex chromosomes because he believed they determined the sex of the fly.
n Normal fruit flies have red eyes.
n Morgan crossed two red eyed parent flies and obtained a white eyed male. In other crosses, he obtained red eyed females, red eyed males and white eyed males.
n Since the white eye color was only present in the male flies, Morgan concluded that eye color was linked to an organisms’ sex.
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n The gene for eye color in fruit flies was located on the sex chromosome, in this case the X chromosome. à Such genes are called sex-linked genes
n Sex-Linked trait: A trait that is carried on one of the Sex Chromosomes (X or Y)
Morgan also found that certain genes on the same chromosome are called “Linked Genes”.
He said that Linked genes get inherited together and not separately as Mendel had proposed (They do not obey Mendel’s Law of Independent assortment.)
n This would account for some differences in ratios of crosses. Instead of getting a 9:3:3:1 expected ratio in a Dihybrid cross, the ratio may be different.
n Morgan also found that genes on the same chromosome that are separated by a great distance will separate as a result of Crossing Over.
Morgan helped to restate Mendel’s Law of Independent Assortment:
LAW OF INDEPENDENT ASSORTMENT IN MODERN TERMS
If crossing over does not occur, genes that are located on the same chromosomes will be inherited together while those on separate chromosomes will assort independently.
Sex-Linked Inheritance
n Certain traits depend on the sex of the parent which carries the trait. The genes for these traits are located on the sex chromosomes, X or Y.
n Transmission of genes which are located on the sex chromosomes is called sex-linked inheritance
n Genes which are located on the X chromosome are called X-linked while those on the Y chromosome are called Y-linked. Most sex linked genes are located on the X chromosome
n Sex-linked traits affect males more often than females
To see how these traits are passed on, let’s do a sample cross:
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Sample Sex-linked cross:
A woman who has normal vision marries a man that is colourblind. What are the possible ratios for their offspring?
Answer Because this is a sex-linked trait we must use the X and Y chromosomes in our cross. Here is how it goes: N = normal n = colorblind PARENTS: XNXN (mother) x XnY (father) RESULTS: Females: All females (XX) will be carriers of the gene, but they will be Normal. Males: All males will be Normal. Let’s do another cross. This time we will cross one of the females with one of the males from above and see what happens.
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Chromosomes and Gene Expression
Chromosome Inactivation
n Males and females produce the same amounts of proteins. This is coded by genes which are located on the X chromosome.
n Females have two X chromosomes in their cells while males have only one X chromosome.
n One of the two female X chromosomes is inactivated and this inactivated chromosome is called a Barr body
Examples of Sex-Linked Traits
n Red-Green Colour blindness n Male Pattern Baldness n Hemophilia n Duchenne Muscular Dystrophy
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1. In humans, the gene for normal blood clotting, H, is dominant to the gene for haemophilia, h. The trait
is sex linked. A woman with normal blood clotting has four children. They are a normal son, a
haemophiliac son, and two normal daughters. The father has normal blood clotting. What is the probable
genotype of each member of the family?
2. In humans, the gene for haemophilia is sex linked and recessive to the gene for normal blood clotting.
What would be the possible ratios for a cross between a haemophiliac male and a normal female? Do
both types of potential crosses.
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3. Calculate the ratio for a cross between a female that is a carrier for the sex-linked trait of glaucoma
where she is crossed with a normal male. Note: Glaucoma is a recessive gene.
4. In fruit flies (Drosophilia), one eye color gene is X-linked, with a recessive white allele and a dominant
red allele. If white-eyed female flies are bred to red-eyed male flies, describe the expected offspring
(assume all parental flies are true-breeding). What results do you expect if you do the reciprocal cross
(reverse the phenotypes of the parent flies)?
5. Earl has normal color vision, while his wife Erma is colorblind. Colorblindness is an X-linked trait, and
the normal allele is dominant to the colorblindness allele. If they have a large family, in what ways should
the colorblindness trait affect their children?
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6. Ethan is colorblind. His wife, Edna, is homozygous for the normal color vision allele. If they have eight
children, how many of them would you expect to be colorblind?
7. Marian’s father is colorblind, as is her maternal grandfather (her mother’s father). Marian herself has
normal color vision. Marion and her husband, Martin, who is also colorblind, have just had their first child,
a son they have named Mickey.
A) What is the probability that this child will be colorblind?
B) Three sources of the colorblindness allele are mentioned in this family. If Mickey is colorblind, from
which of these three men (Marian’s grandfather, Marion’s father, or Martin) did he inherit the allele
from?
C) If Martin were not colorblind, how would this affect the prediction about Mickey?
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8. In cats, there is a coat color gene located on the X chromosome. This gene has two alleles – orange
and black. A heterozygous cat has tortoiseshell color (a splotchy mixture of orange and black). Predict
the genotypic and phenotypic frequencies among the offspring of the following crosses. Pay careful
attention to the genders of the offspring.
A) Black female x Orange male
B) Orange female x Black male
C) Tortoiseshell female x Black male
D) Tortoiseshell female x Orange male
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Polygenic Inheritance
n The inactivation of the X chromosome in different cells is random. So different cells may have a different X chromosomes activated
n This is responsible for producing
o The tortoiseshell coat color in female cats
o Different lengths of ear corn in crops
n Most traits are controlled by one gene, however, some traits are controlled by more than one gene, this is called polygenic inheritance
n Polygenic genes cause a range of variation in individuals called continuous variation
Polygenic Traits in Humans
n Height
n Skin Colour
n Hair
n Eye Colour
Modifier Genes
n Modifier genes – Genes that work with other genes to control the expression of a particular trait.
n In humans, modifier genes help control the trait of eye colour. n In this case, modifier genes influence the level of melanin present in the human
eye to provide a range of eye colors from blue to brown. n Really there should only be TWO types of eye colour in humans:
§ Brown (dominant) § Blue (recessive)
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1. In fruit flies, the gene for wing shape has an unusual allele called ‘curly’ (designated ‘Cy’). The normal
(wild type) allele is designated ‘cy’. A fly homozygous for cy (cy cy) has normal, straight wings. The
heterozygote (Cy cy) has wings which curl up on the ends (and, incidentally, can’t really fly). The
homozygotes for the Cy allele (Cy Cy) never hatches out of the egg. In other words, this allele is lethal
in the homozygous condition. If two curly winged flies are mated, and the female lays 100 eggs, predict
the following, showing appropriate work: Perform this cross (HINT: it’s a monohybrid)
a. How many eggs will produce living offspring?
b. How many straight winged flies do you expect among the living offspring?
c. What percentage of the living offspring do you expect to be curly winged like the parents?
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2. In cats, black color is dominant to a special, temperature-sensitive albino gene which produces cats
with dark legs, faces and tails (Siamese cats, in case you don’t recognize it!). A short haired (dominant)
Siamese colored female is bred to a long-haired black male. They have eight kittens:
2 black, short haired
2 black, long haired
2 Siamese, short haired
2 Siamese, long haired
What were the genotypes of the two parents?
3. Elizabeth is married to John, and they have four children. Elizabeth has a straight nose (recessive) and
is able to roll her tongue (dominant). John is also able to roll his tongue, but he has a convex (Roman)
nose (dominant). Of their four children, Ellen is just like her father, and Dan is just like his mother. The
other children – Anne, who has a convex nose, and Peter, who has a straight nose – are unable to roll
their tongues. Please answer the following questions about this family.
What are the genotypes of Elizabeth and John?
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4. When a male pig from a line of true-breeding (homozygous) black, solid-hooved pigs was crossed to a
female from a breed (homozygous) of red, cloven-hooved pigs, their several offspring all looked alike
with regard to color and hooves. These offspring were all mated to members of the same breed as their
red, cloven-hooved mother pig. The offspring from this final cross were:
11 black, cloven-hooved; 8 black, solid-hooved; 14 red, cloven-hooved; 10 red, solid-hooved
For each of these two genes (coat color and hoof type) determine which allele is the dominant one.
Explain your reasoning. What were the phenotypes of the offspring produced by the first mating in this
problem?
5. In garden peas, long stems are dominant to short stems, and yellow seeds are dominant to green
seeds. 100 long/yellow pea plants, all of which had one short/green parent, are interbred (bred to each
other). 1600 offspring result. Answer the following questions:
a. Assuming that these two genes are unlinked, about how many long/green pea plants would you expect
to find among the offspring? __________
b. What ratio of yellow to green seed color would you expect among the offspring? ________
c. What would you expect the overall phenotypic ratio among the 1600 offspring to be?
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Genetics – the branch of biology dealing with the principles of variation and inheritance in organisms; how traits are passed on from generation to generation.
Traits – distinguishable characteristics or phenotypic features of an individual ex. Eye color, height, hair color.
Heredity – the passing of genetic traits such as the color of eyes or hair from one generation to the next resulting in similarities between members of one family or strain.
Blend theory – an early theory in genetics that states that factors from parents were blended in their offspring. This theory, however, was unable to explain the appearance or disappearance of distinct traits from one generation to another.
P generation – the designation for the parent generation.
F1 generation – the first filial generation, offspring from the cross (mating) of the P generation.
F2 generation – the second filial generation, offspring from the cross of the F1 generation.
Unit characters – a term describing Mendel’s “factors” of inheritance (genes), which are inherited as independent units.
Gene – a specific sequence of DNA that guides the expression of a particular trait and can be passed on to an offspring.
Allele – alternate form of a gene.
Unit theory – a term describing Mendel’s law of inheritance, from his discovery that genes (which he called “factors”) are inherited as independent units.
Dominant – type of trait in which the characteristic is always expressed, or appears, in an individual.
Recessive – having an allele that is latent (present but inactive) and is therefore not usually expressed unless there is no dominant allele present.
Purebred – having descended from ancestors of a distinct type or breed. Purebred organisms in a given species or variety all share similar traits.
Hybrid – an organism heterozygous for a trait.
Homozygous – describes an individual with two alleles at one locus that are identical (two dominant or recessive alleles for one trait).
Heterozygous – describes an individual with two different alleles at a locus (one dominant and one recessive allele for a single traits).
Genotype – genetic makeup of an organism; remains constant throughout an individual’s life. Usually indicated by the combination of letters in a Punnett square.
Phenotype – the physical and physiological traits of an organism.
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Punnett Square – simple grid used to illustrate all possible combinations of gametes from a given set of parents.
Product rule – rule that states that the probability, or chance, that two or more independent events will occur together is the product of their individual probabilities occurring alone Ex. When flipping a coin, the chance of getting heads twice in a row is ½ x ½ = ¼
Monohybrid cross (or one factor cross) – is a cross which involves only one trait.
Principle of dominance – when individuals of contrasting traits are crossed, the offspring will express only the dominant trait.
Law of segregation – Mendel’s first law of inheritance, in which the hereditary traits are determined by the pair of alleles from each parent. These alleles are separated during gamete formation, giving each offspring only one allele from each parent.
Dihybrid cross – cross of two heterozygous individuals for two different traits.
Law of independent assortment – Mendel’s second law of inheritance, stating that the inheritance of alleles for one trait does not affect the inheritance of alleles for another trait.
Incomplete dominance – Blending of the two traits of two different alleles (pink flowers instead of red or white)
Co-dominance – Two alleles may be expressed equally. The situation occurs when two different alleles for a trait are both dominant.
Multiple Alleles – Pattern of inheritance when a gene may have more than two alleles for any given trait for example, human blood type is controlled by 3 alleles.
Polygenic Inheritance – Is a condition in which traits are governed by more than one gene (skin/eye color).
Test cross – cross of an individual with unknown genotype with a homozygous recessive individual; used as a method to identify an unknown genotype.
Pedigree – diagram that illustrates the genetic relationships among a group of individuals.
Sex-linked inheritance – the transfer of genes on the X or Y chromosome from one generation to the next.
Barr body – the inactivated X chromosome.
Polygenic inheritance – pattern of inheritance in which a trait is controlled by more than one gene.
Continuous variation – range of variation in one trait resulting from the protein products produced by many genes.
Modifier genes – genes that work along with other genes to control the expression of a trait.
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