Sex Chromosomes Sex Chromosomes Xlinked Traits Possible genotypes

Sex Chromosomes

Sex Chromosomes

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

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

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

X-linked Dominant Inheritance: Congenital Generalized Hypertrichosis Figure 6. 10

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

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

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

X Chromosome Inactivation

Fur Color in Tortoiseshell Cats Orange fur Black fur

Manifesting Heterozygote A carrier of an X-linked trait who expresses the phenotype l If a female is heterozygous for a recessive trait, the dominant allele will usually mask the recessive allele… l l l 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

Multifactorial Traits Genes and the Environment

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

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

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

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

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

Concordance • The percentage of 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

Review