Sex Chromosomes

1. Sex Chromosomes

2. Sex Chromosomes

3. Possible genotypes X+Y  Hemizygouswild type male XmY Hemizygous mutant male X+X+ Homozyogus wild female X+Xm Heterozygous female carrier XmXm Homozygous mutant female X-linked Traits

4. X-linked Recessive Inheritance • Always expressed in hemizygous males • Female homozygotes show the trait but female heterozygotes do not • Affected males: Inherited from affected or heterozygous mother • Affected females: affected fathers and affected or heterozygous mothers

6. X-linked Dominant Inheritance • Expressed with one copy • Males are often more severely affected • Typically associated with miscarriage or lethality in males • Passed from father to all his daughters but none of his sons

7. X-linked Dominant Inheritance: Congenital Generalized Hypertrichosis Figure 6.10

8. Sex-limited traits • A trait that affects a structure or function of the body that is present in only one of the sexes. • May be X-linked or autosomal • Example: A gene affecting milk production will not have an effect in males. However, males can carry and pass on the trait. • Why do men have nipples anyway???

9. Sex-influenced traits • An allele is dominant in one sex but recessive in the other sex. • May be X-linked or autosomal • Due to hormonal interactions • Men have testosterone • Women have estrogen

10. X-inactivation • Females “turn off” one of their X chromosomes in each cell • In order to be more equal to males who only have one X chromosome • The X chromosome turned off in each cell is random

11. X Chromosome Inactivation

12. Fur Color in Tortoiseshell Cats Orange fur Black fur

13. Manifesting Heterozygote • A carrier of an X-linked trait who expresses the phenotype • If a female is heterozygous for a recessive trait, the dominant allele will usually mask the recessive allele… • Unless the dominant allele is on the X chromosome that was inactivated • Some cells will express the trait and others will not, depending on which X chromosome is inactivated

14. Multifactorial Traits Genes and the Environment

15. Polygenic Traits • A trait is influenced by more than one gene • May be multifactorial (influenced by environment)

16. Polygenic Traits are Continuously Varying • Each gene in the polygenic trait contributes to the phenotype to a varying degree • Example: Height • Polygenic (influenced by multiple genes) • Continuous

17. Pure Polygenic Trait -Eye Color Figure 7.3 • The number of human eye color genes is unknown • Analysis will probably reveal many genes • Mice have more than 60 eye color genes

18. Analyzing Multifactorial Traits • Difficult, requires multiple techniques • Use human genome sequences, population, and family studies • The frequency in a specific population = Empiric risk • The amount of inheritance due to genes = Heritability

19. Dizygotic twins: Shared environment and 50% of genes Monozygotic twins: Identical genotype, and shared environment Twins raised apart: Shared genotype but not environment Adopted individuals: Shared environment but not genes Separating Genes and Environment

20. Concordance • The percentageof pairs in which both twins express the trait • Used to determine heritability • Assumes both types of twins share similar environments • MZ twins often share more similar environments

21. Review