NonMendelian Genetics

NonMendelian Genetics

NonMendelian Genetics

Chapter 14: Mendel and the Gene IdeaChapter 14: Mendel and the Gene Idea

Complex patterns of inheritance

Complex patterns of inheritance

The relationship between genotype and phenotype is rarely as simple as in Mendelian inheritance (controlled by dominant and recessive paired alleles)

Principles of segregation and independent assortment apply to more complex patterns of inheritance

Inheritance may deviate from simple Mendelian patterns in the following situations:Alleles are not completely dominant or recessiveA gene has more than two allelesA gene produces multiple phenotypes

The relationship between genotype and phenotype is rarely as simple as in Mendelian inheritance (controlled by dominant and recessive paired alleles)

Principles of segregation and independent assortment apply to more complex patterns of inheritance

Inheritance may deviate from simple Mendelian patterns in the following situations:Alleles are not completely dominant or recessiveA gene has more than two allelesA gene produces multiple phenotypes

Incomplete dominanceIncomplete dominance In complete dominance,

heterozygous and homozygous dominant individuals have the same phenotype

With incomplete dominance, the phenotype of the heterozygous is intermediate between those of the two homozygotes

This intermediate occurs because neither allele of the pair is completely dominant

In complete dominance, heterozygous and homozygous dominant individuals have the same phenotype

With incomplete dominance, the phenotype of the heterozygous is intermediate between those of the two homozygotes

This intermediate occurs because neither allele of the pair is completely dominant

Incomplete DominanceIncomplete Dominance If you cross a white flower with a red flower that

exhibit incomplete dominance the first generation (heterozygotes) will be all pink.

If you cross two of those heterozygotes you will get 1 red, 2 pink, 1 white flowers. (1:2:1 phenotypic ratio)

If you cross a white flower with a red flower that exhibit incomplete dominance the first generation (heterozygotes) will be all pink.

If you cross two of those heterozygotes you will get 1 red, 2 pink, 1 white flowers. (1:2:1 phenotypic ratio)

We can still use the Punnett Square to solve problems involving incomplete dominance.

The trick is to recognize when you are dealing with a question involving incomplete dominance.

There are two steps to this: 1) Notice that the offspring is showing a 3rd

phenotype. The parents each have one, and the offspring are different from the parents.

2) Notice that the trait in the offspring is a blend (mixing) of the parental traits.

We can still use the Punnett Square to solve problems involving incomplete dominance.

The trick is to recognize when you are dealing with a question involving incomplete dominance.

There are two steps to this: 1) Notice that the offspring is showing a 3rd

phenotype. The parents each have one, and the offspring are different from the parents.

2) Notice that the trait in the offspring is a blend (mixing) of the parental traits.

Incomplete dominance genetic problems

Incomplete dominance genetic problems

Incomplete Dominance Questions


1. A cross between a black bird & a white bird produces offspring that are grey. The color of birds is determined by just two alleles. a) What are the genotypes of the parent birds in the

original cross? BB = black, BW= grey, WW = whiteBB x WW

b) What is/are the genotype(s) of the grey offspring? BW

c) What would be the phenotypic ratios of offspring produced by two grey birds?

1 black, 2 grey and 1 white

1. A cross between a black bird & a white bird produces offspring that are grey. The color of birds is determined by just two alleles. a) What are the genotypes of the parent birds in the

original cross? BB = black, BW= grey, WW = whiteBB x WW

b) What is/are the genotype(s) of the grey offspring? BW

c) What would be the phenotypic ratios of offspring produced by two grey birds?

1 black, 2 grey and 1 white

B W

B BB BW

W BW WW



2. 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? RY

2. 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? RY

CodominanceCodominance In codominance, two

dominant alleles affect the phenotype in separate, distinguishable ways

Codominant alleles cause the phenotypes of both homozygotes to be produced in heterozygote individuals.

In codominance both alleles are expressed.

For example, red cows crossed with white will generate roan cows. Roan refers to cows that have red coats with white blotches.

In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

Codominant alleles cause the phenotypes of both homozygotes to be produced in heterozygote individuals.

In codominance both alleles are expressed.

For example, red cows crossed with white will generate roan cows. Roan refers to cows that have red coats with white blotches.

CodominanceCodominanceThe genetic gist to codominance is pretty much the

same as incomplete dominance.A hybrid organism shows a third phenotype --- not

the usual "dominant" one & not the "recessive" one.With incomplete dominance we get a blending of

the dominant & recessive traits so that the third phenotype is something in the middle (red x white = pink).

In codominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms.

red x white ---> red & white spotted

The genetic gist to codominance is pretty much the same as incomplete dominance.

A hybrid organism shows a third phenotype --- not the usual "dominant" one & not the "recessive" one.

With incomplete dominance we get a blending of the dominant & recessive traits so that the third phenotype is something in the middle (red x white = pink).

In codominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms.

red x white ---> red & white spotted

Codominance Punnett Squares

Codominance Punnett Squares

Some texts use letters & superscripts when dealing with codominance.

Others use different letters, noting the type of nonMendelian cross.Let’s use the second method

for our example R = allele for red flowers W = allele for white flowersred x white --> red & white

spotted flowers RR x WW ----> 100% RWThe symbols you choose to use

don't matter, in the end you end up with hybrid organisms, and rather than one trait (allele) dominating the other, both traits appear together in the phenotype.

Some texts use letters & superscripts when dealing with codominance.

Others use different letters, noting the type of nonMendelian cross.Let’s use the second method

for our example R = allele for red flowers W = allele for white flowersred x white --> red & white

spotted flowers RR x WW ----> 100% RWThe symbols you choose to use

don't matter, in the end you end up with hybrid organisms, and rather than one trait (allele) dominating the other, both traits appear together in the phenotype.

RWRW

RWRW

R R

W

W

Codominance QuestionsCodominance Questions1. Predict the phenotypic ratios of offspring

when a homozygous white cow is crossed with a roan bull. White = WW, roan = RW, red = RRWW x RW = 1/2 white and 1/2 roan

2. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). a) What pattern of inheritance does this illustrate?

Why? Codominance; both showb) What percent of kittens would have tan fur if a

tabby cat is crossed with a black cat?TT=tan, TB = tabby, BB= blackTB x BB = 1/2 tabby and 1/2 black0%

1. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull. White = WW, roan = RW, red = RRWW x RW = 1/2 white and 1/2 roan

2. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). a) What pattern of inheritance does this illustrate?

Why? Codominance; both showb) What percent of kittens would have tan fur if a

tabby cat is crossed with a black cat?TT=tan, TB = tabby, BB= blackTB x BB = 1/2 tabby and 1/2 black0%

W W

R RW RW

W WW WW

T B

B TB BB

B TB BB

Multiple AllelesMultiple Alleles

It is common for more than two alleles to control a trait in a population

Traits controlled by more than two alleles are said to have multiple alleles

A diploid individual can possess only two alleles of each gene

It is common for more than two alleles to control a trait in a population

Traits controlled by more than two alleles are said to have multiple alleles

A diploid individual can possess only two alleles of each gene

Multiple AllelesMultiple Alleles

The number of alleles for any particular trait is not limited to four, there are instances in which more than 100 alleles are known to exist for a single trait

Multiple Alleles & Blood Types

Multiple Alleles & Blood Types

Multiple Alleles govern blood typeHuman blood types are

determined by the presence or absence of certain molecules on the surfaces of red blood cells called antigens

As the determinant of blood type the gene I has three alleles: IA, IB, and iWritten A, B, and OIA (or A) allele produces

antigen A IB (or B) allele produces

antigen Bi (or O) produces no antigensA & B codominant, but both

dominant to O

Multiple Alleles govern blood typeHuman blood types are

determined by the presence or absence of certain molecules on the surfaces of red blood cells called antigens

As the determinant of blood type the gene I has three alleles: IA, IB, and iWritten A, B, and OIA (or A) allele produces

antigen A IB (or B) allele produces

antigen Bi (or O) produces no antigensA & B codominant, but both

dominant to O

Importance of Blood Typing

Importance of Blood Typing

Incompatible blood types could clump together, causing death.

Disputed parentageExample: If a child has type AB blood and

its mother has type A, a man with type O blood could not be the father.

Why?

Incompatible blood types could clump together, causing death.

Disputed parentageExample: If a child has type AB blood and

its mother has type A, a man with type O blood could not be the father.

Why?

Blood Typing PracticeBlood Typing Practice1. A woman with Type O blood and a man who is Type AB

have are expecting a child. What are the possible blood types of the kid? • A, B

2. What are the possible blood types of a child who's parents are both heterozygous for "B" blood type? • B,O

3. What are the chances of a woman with Type AB and a man with Type A having a child with Type O? • None

4. A test was done to determine the biological father of a child.The child's blood Type is A and the mother's is B. Man #1 has a blood type of O, & Man #2 has blood type AB. Which man is the biological father?• Man #2

1. A woman with Type O blood and a man who is Type AB have are expecting a child. What are the possible blood types of the kid? • A, B

2. What are the possible blood types of a child who's parents are both heterozygous for "B" blood type? • B,O

3. What are the chances of a woman with Type AB and a man with Type A having a child with Type O? • None

4. A test was done to determine the biological father of a child.The child's blood Type is A and the mother's is B. Man #1 has a blood type of O, & Man #2 has blood type AB. Which man is the biological father?• Man #2

PleiotropyPleiotropy

Most genes have multiple phenotypic effects, a property called pleiotropy

For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

In the garden pea, gene for flower color also affects color of seed coat

Most genes have multiple phenotypic effects, a property called pleiotropy

For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

In the garden pea, gene for flower color also affects color of seed coat

EpistasisEpistasis In epistasis, a gene at one

locus alters the phenotypic expression of a gene at a second locus

For example, in mice and many other mammals, coat color depends on two genes

One gene determines the pigment color (B for black and b for brown)

The other gene (C for color and c for no color) determines whether the pigment will be deposited in the hairDominance = B masking bEpistasis = cc masking BB

or Bb or bb

In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus

For example, in mice and many other mammals, coat color depends on two genes

One gene determines the pigment color (B for black and b for brown)

The other gene (C for color and c for no color) determines whether the pigment will be deposited in the hairDominance = B masking bEpistasis = cc masking BB

or Bb or bb

Polygenic inheritance

poly = “many” ; genic = “genes”

Polygenic inheritance

poly = “many” ; genic = “genes”

More than one gene pair contributes to a phenotype

Effects of dominant alleles are additiveMore dominant genes = increased effectNumber of dominant determines

phenotypeHair, eye and skin color (as well as height)

are polygenic traitsMany disorders may be polygenic

Cleft palate, club foot, diabetes, schizophrenia, allergies, cancer

More than one gene pair contributes to a phenotype

Effects of dominant alleles are additiveMore dominant genes = increased effectNumber of dominant determines

phenotypeHair, eye and skin color (as well as height)

are polygenic traitsMany disorders may be polygenic

Cleft palate, club foot, diabetes, schizophrenia, allergies, cancer

Skin color exampleSkin color example If skin color was related to 3 gene

pairsDominant gene A, B or C

produces pigmentIncompletely dominant to a, b or

cSo # of dominant genes

determines how much pigment is produced

AABBCC = lots of pigmentAaBbCc = middle range of

pigmentaabbcc = very little pigment2 heterozygotes (AaBbCc) could

have a child with any pigment range

If skin color was related to 3 gene pairsDominant gene A, B or C

produces pigmentIncompletely dominant to a, b or

cSo # of dominant genes

determines how much pigment is produced

AABBCC = lots of pigmentAaBbCc = middle range of

pigmentaabbcc = very little pigment2 heterozygotes (AaBbCc) could

have a child with any pigment range

Environmental Influences

Environmental Influences

Genes are also influenced by the environment

Temperature and Siamese cats

The darker colors on the extremities are due to a cooler body temperature

Gene that codes for production of the pigment in the Siamese cat only functions under cooler conditions

Many diseases, such as heart disease and cancer, have both genetic and environmental components

Genes are also influenced by the environment

Temperature and Siamese cats

The darker colors on the extremities are due to a cooler body temperature

Gene that codes for production of the pigment in the Siamese cat only functions under cooler conditions

Many diseases, such as heart disease and cancer, have both genetic and environmental components

PedigreePedigree A pedigree is a family tree that

describes the interrelationships of parents and children across generations Inheritance patterns of

particular traits can be traced

Can also be used to make predictions about future offspring

Many genetic disorders are inherited in a recessive manner

Recessively inherited disorders show up only in individuals homozygous for the allele

Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal

A pedigree is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of

particular traits can be traced

Can also be used to make predictions about future offspring

Many genetic disorders are inherited in a recessive manner

Recessively inherited disorders show up only in individuals homozygous for the allele

Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal

Pedigree SymbolsPedigree Symbols

AlbinismAlbinism Albinism is a recessive

condition characterized by a lack of pigmentation in skin and hair

If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low

Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele

Most societies and cultures have laws or taboos against marriages between close relatives

Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair

If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low

Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele

Most societies and cultures have laws or taboos against marriages between close relatives

Cystic FibrosisCystic FibrosisRecessive conditionCystic fibrosis is the most

common lethal genetic disease in the US, striking one out of every 2,500 people of European descent

The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes

Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

Recessive conditionCystic fibrosis is the most

common lethal genetic disease in the US, striking one out of every 2,500 people of European descent

The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes

Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

Sickle-cell diseaseSickle-cell disease

Recessive conditionSickle-cell disease

affects one out of 400 African-Americans

The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells

Symptoms include physical weakness, pain, organ damage, and even paralysis

Recessive conditionSickle-cell disease

affects one out of 400 African-Americans

The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells

Symptoms include physical weakness, pain, organ damage, and even paralysis

Dominant Genetic Diseases

Dominant Genetic Diseases

Some human disorders are caused by dominant alleles

Dominant alleles that cause a lethal disease are rare and arise by mutation

Achondroplasia is a form of dwarfism caused by a rare dominant allele

Huntington’s disease is a degenerative disease of the nervous system caused by a dominant alleleThe disease has no obvious

phenotypic effects until the individual is about 35 to 40 years of age

Some human disorders are caused by dominant alleles

Dominant alleles that cause a lethal disease are rare and arise by mutation

Achondroplasia is a form of dwarfism caused by a rare dominant allele

Huntington’s disease is a degenerative disease of the nervous system caused by a dominant alleleThe disease has no obvious

phenotypic effects until the individual is about 35 to 40 years of age

Genetic TestsGenetic Tests

There are many genetic diseases that exist (way beyond the scope of what we will discuss)

Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease

Using family histories, they help couples determine the odds that their children will have genetic disorders

For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately

There are many genetic diseases that exist (way beyond the scope of what we will discuss)

Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease

Using family histories, they help couples determine the odds that their children will have genetic disorders

For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately

AmniocentesisAmniocentesis

In amniocentesis, a long thin needle is used to remove amniotic fluid

The amniotic fluid contains fetal cells, which can be tested for genetic diseases

The DNA from fetal cells is karyotyped

In amniocentesis, a long thin needle is used to remove amniotic fluid

The amniotic fluid contains fetal cells, which can be tested for genetic diseases


Chorionic Villus SamplingChorionic Villus Sampling

In chorionic villus sampling (CVS), a sample of the chorionic villus (developing placenta) is removed and tested

The chorionic villus cells contain the same genetic material as the fetus, making them fetal cells


In chorionic villus sampling (CVS), a sample of the chorionic villus (developing placenta) is removed and tested

The chorionic villus cells contain the same genetic material as the fetus, making them fetal cells


KaryotypesKaryotypes

Karyotypes (picture of chromosomes arrested during mitosis) are prepared, which determines:# of chromosomesSex of individualExtra or missing

pieces of chromosomes

Karyotypes (picture of chromosomes arrested during mitosis) are prepared, which determines:# of chromosomesSex of individualExtra or missing

pieces of chromosomes

Other Genetic TestsOther Genetic Tests

Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero

Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the USPhenylketonuria (PKU)Congential

Hypothyroidism

Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero

Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the USPhenylketonuria (PKU)Congential

Hypothyroidism

Review QuestionsReview Questions1. Name 3 examples of when inheritance patterns may not

follow Mendelian rules.2. Explain, identify, and solve genetics problems involving

incomplete dominance, codominance, & multiple alleles.3. Complete genetics problems involving blood types.4. Explain, differentiate between, and complete nontraditional

genetics problems involving pleiotropy, epistasis, and polygenic inheritance.

5. Explain the effect of the environment on the expression of our genes.

6. Define and analyze a pedigree in order to answer inheritance questions.

7. Identify the most common pedigree symbols.8. Identify the inheritance patterns and major characteristics

of the following genetic conditions: albinism, cystic fibrosis, sickle-cell disease, achondroplasia, & Huntington’s disease.

9. Explain the purpose, benefits, and risks of genetic testing.10. Differentiate between amniocentesis and chorionic villus

sampling.11. Explain the purpose and use of a karyotype.12. List 3 pieces of information that can be obtained from a

karyotype.

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NonMendelian Genetics