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Genes on the X Chromosome:

II. III. XY. IV. or. XX. Genes on the X Chromosome:. *1909 Thomas Hunt Morgan. *Sex chromosomes *Autosomes. Example: In Drosophila and all mammals sex chromosomes designated as X and Y XX=female XY=male. Genes on the X Chromosome con’t:. *X-linkage. X-linkage in Drosophila :

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Genes on the X Chromosome:

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  1. II III XY IV or XX Genes on the X Chromosome: *1909 Thomas Hunt Morgan *Sex chromosomes *Autosomes Example: In Drosophila and all mammals sex chromosomes designated as X and Y XX=female XY=male

  2. Genes on the X Chromosome con’t: *X-linkage X-linkage in Drosophila: white mutation (eyes)

  3. Sex-limited Inheritance: *Sex-limited trait *holandric genes: genes on the Y chromosome *autosomal genes Example: milk production in mammals; L=lots, l=little

  4. 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

  5. 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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