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 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 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
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 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 I B i. No, Mr. Chan cannot be the child’s father. His genotype is I A I B, 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 I A I A (AA) or I A i (AO) Wife could be I B I B (BB) or I B i (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 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.