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II. III. XY. IV. or. XX. Sex Linkage. *1909 Thomas Hunt Morgan. *Sex chromosomes *Autosomes. Example: In Drosophila and all mammals sex chromosomes designated as X and Y XX=female XY=male. sex-linkage - transmission and expression of genes on sex chromosomes.

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II

III

XY

IV

or

XX

Sex Linkage

*1909 Thomas Hunt Morgan

*Sex chromosomes

*Autosomes

Example:

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

XX=female

XY=male

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


Modifications of Mendelian Ratios

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

X-linkage in Drosophila:

white mutation (eyes)


Sex-limited Inheritance:

*Sex-limited trait

*holandric genes: genes on the Y chromosome

*autosomal genes

Example: milk production in mammals; L=lots, l=little


Sex-influenced Inheritance:

*Sex-influenced trait

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



Phenotypic Expression:

Gene expression often governed by genotype and environment

*Penetrance

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

*Expressivity

*Temperature

*Onset of genetic expression


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