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 copyMales are often more severely affectedTypically associated with miscarriage or

lethality in malesPassed 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 autosomalExample: 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 autosomalDue to hormonal interactions

Men have testosteroneWomen 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 chromosomeThe 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

• The number of human eye color genes is unknown

• Analysis will probably reveal many genes

• Mice have more than 60 eye color genes

Figure 7.3

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