Sex Linkage

*Sex chromosomes*Autosomes

Example: In Drosophila and all mammals sex chromosomes designated as X and Y

XX=femaleXY=male

*1909 Thomas Hunt MorganII III IV

XX

XY

or

sex-linkage - transmission and expression of genes on sex chromosomes

X-linkage - transmission and expression of genes on the X chromosome

X-linkage in Drosophila:

white mutation (eyes)

Modifications of Mendelian Ratios

Sex-limited Inheritance:*Sex-limited trait

*holandric genes: genes on the Y chromosome

*autosomal genesExample: milk production in mammals; L=lots, l=little

*Sex-influenced trait

Sex-influenced Inheritance:

Examples:

*cleft palate in humans*horns in sheep *pattern baldness in humans

Summary:

Sex-linked on X or Y sex-chromosome

Sex-limited all or none expression by sex

Sex-influenced genotype + sex determines phenotype

*Phenotype depends on environment and genes

Phenotypic Expression:

*Penetrance

*Expressivity

Gene expression often governed by genotype and environment

If 9/10 of individuals carrying an allele express the trait, the trait is said to be 90% penetrant

*Onset of genetic expression

*Temperature