Genetics: Complex Inheritance, Sex Linkage, X-Inactivation AP Biology Unit 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 is pleiotropy (where one gene affects several different phenotypes) ex. PKU

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 Pedigrees Widow’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 XYXX XYX 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 –X A, X a

Red-Green Colorblindness Gene that controls this (opsin gene) is on the X chromosome Colorblindness is caused by a recessive allele (mutation in the opsin gene) 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.

Sex-limited Trait Trait whose expression depends on the sex of the individual Not found on the X or Y chromosome- NOT the same thing as sex-linked trait. Ex. Milk production in females, pattern baldness in males (triggered by hormones)

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 #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 X B Y, the woman is X B X b. 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.