Sex Chromosomes

Sex Chromosomes

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

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

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

X-linked Dominant Inheritance: Congenital Generalized Hypertrichosis Figure 6.10

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???

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

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

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 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

Multifactorial Traits Genes and the Environment

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

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

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 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

Separating Genes and Environment 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

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 Mitochondrial

X-linked Recessive