1. Genetics: Complex Inheritance, Sex Linkage, X-Inactivation
AP Biology
Unit 3

2. Incomplete Dominance
Heterozygous phenotype is a blend of the 2 homozygous phenotypes
Ex. Red flower crossed with white flower  heterozygous flower is pink

3. 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

4. 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)

5. Nature vs. Nurture
Virtually all human diseases have some genetic component

6. 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.

7. Sample Pedigrees What is the grandfather’s genotype? Ww
Widow’s Peak = Dominant Trait
W = widow’s peak
What is the grandfather’s genotype? Ww

8. Sample Pedigrees Attached earlobes = Recessive Trait F = unattached
f = attached
What is the genotype of these grandparents?
Both Ff

10. 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

11. 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

12. 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.

14. 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

15. Example: Calico (Tortoiseshell) cats
animation

16. 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

17. Practice Problem #2
A couple (both unaffected) has a son with cystic fibrosis (a recessive disease; allele is represented as “c”)
What is the genotype of the son?
What are the genotypes for the couple?
What are the chances the couple’s second child will also have cystic fibrosis?
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?

18. Answer #2 Son’s genotype is cc
Both parents are carriers (Heterozygotes) = Cc
25% chance
0% chance– but all children will be carriers

19. 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?

20. 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.

21. 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?

22. 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)

23. 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?

24. 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.

25. 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

26. 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.