Genetics:Genetics:Complex Inheritance, Complex Inheritance, Sex Linkage, Sex Linkage, X-InactivationX-Inactivation

AP BiologyUnit 3

Incomplete Dominance

• Heterozygous phenotype is a blend of the 2 homozygous phenotypes

• Ex. Red flower crossed with white flower heterozygous flower is pink

Epistasis

• When one gene product affects the expression of another gene.

• B and b are fur color alleles

• bb = brown fur

• Fur color will only be expressed if the C gene is also present

Polygenic Inheritance

• Two or more genes work together to create a single phenotype

• Eye color, skin color are good examples

• Opposite of pleiotropy (where one gene affects several different phenotypes)

• Nature vs. Nurture

• Virtually all human diseases have some genetic component

Pedigrees• Used to trace the

genotypes for a particular trait in a family

• Can help determine the probability that future offspring will have a trait.

Sample PedigreesWidow’s Peak = Dominant Trait W = widow’s peak

• What is the grandfather’s genotype?

• Ww

Sample Pedigrees

• Attached earlobes = Recessive Trait

F = unattached f = attached• What is the genotype of

these grandparents?• Both Ff

Sex Linkage

• When a trait is carried on the X or Y chromosomes, it is called a sex-linked trait

• Don’t confuse this with linked genes = when 2 genes are on the same chromosome

XY XX

X Y X

XX XY

Sex-linked genes and Punnett Squares

• You have to include the X and Y chromosomes in the Punnett Square

• Superscripts on the X and Y denote which allele is present

Red-Green Colorblindness

• Gene that controls this is on the X chromosome

• Who is more likely to be color blind– men or women?– Men: only 1 X chromosome – if they have

the recessive allele they don’t have another X to make up for it.

X-inactivation in female mammals

• In females, one of the two X chromosomes in each cell becomes inactive during embryonic development

• Why would one X chromosome inactivate itself in females? – Cells of females and males would have same

effective dose of genes on the X chromosome

• Inactive X chromosomes are called Barr bodies

Example: Calico (Tortoiseshell) cats

• animation

Example in humans

• Anhidrotic dysplasia

• X linked mutation prevents the development of sweat glands

• A woman who is heterozygous will have patches of normal skin and patches of skin without sweat glands

• Difficulty controlling body temperature

Practice Problem #2• A couple (both unaffected) has a son with cystic

fibrosis (a recessive disease; allele is represented as “c”)

a) What is the genotype of the son?

b) What are the genotypes for the couple?

c) What are the chances the couple’s second child will also have cystic fibrosis?

d) If the son marries a woman who does not carry the cystic fibrosis allele, what % chance is there that their children will have cystic fibrosis?

Answer #2

a) Son’s genotype is cc

b) Both parents are carriers (Heterozygotes) = Cc

c) 25% chance

d) 0% chance– but all children will be carriers

Practice Problem #3

• Mr. Chan’s former girlfriend claims that he is the father of her child. The child has O type blood. The child’s mother has B type blood. After testing a sample of Mr. Chan’s blood, you find that he has AB type blood.

• What is the child’s mother’s genotype?

• Could Mr. Chan be the father of the baby?

Answer #3

• Since the child has O type blood, his/her genotype is ii. Since the mother has to give one allele to the baby, her genotype must be IBi.

• No, Mr. Chan cannot be the child’s father. His genotype is IAIB, so he can’t have a child who has an ii genotype.

Practice Problem #4

• A husband has blood type A

• A wife has blood type B

• Daughter has AB blood

• What are the possible blood types (phenotypes) for their son?

Answer #4

• Husband could be IAIA (AA) or IAi (AO)

• Wife could be IBIB (BB) or IBi (BO)

• Son could have any blood type (A, B, AB, or O)

Practice Problem #5

• A normal (not colorblind male) marries a woman who is a carrier for the colorblindness allele

• What are the chances their son will be color blind?

• What are the chances their daughter will be colorblind?

Answer #5

• The man is XBY, the woman is XBXb.

• 50% chance that son will be colorblind

• 0% chance that daughter will be colorblind, but she could be a carrier.

Practice Problem #6• Is this trait on the X or Y

chromosome?• Is it dominant or recessive?• If individual A marries an

unaffected male, what are the % chances her children will have the trait?

• If individual B marries an unaffected woman, what are the chances their children will have the trait?

B

A

Answer #6

• Trait must be on the X chromosome (X*). only men would have it if on Y chromosome.

• Dominant trait– determine the genotypes of males look at their mother’s to help determine recessive or dominant.

• Individual A must be X*X (since her mother was unaffected). 50% chance children will have trait (girls or boys).

• Individual B must be X*Y. All daughters will have trait, none of sons will.