Sex-Linked Traits Most traits are controlled by at least 2 genes, one on each homologue The exception are traits controlled by genes on the sex chromosomes Some genes on the X are not on the Y In males, the trait is only controlled by 1 gene Males are not homozygous or heterozygous, by are called hemizygous (only 1 chromosome has the gene)

Sex-Linked Traits Males either have a dominant or recessive allele; no heterozygous genotypes Males generally show the recessive traits more (50% to 33%) Male: H or h Female: HH, Hh, hh Types of disorders found on the X chromosome: Hemophilia – blood doesn’t clot Color-blindness – can’t see certain colors (red - green) Duchene Muscular Dystrophy – affects cardiac/skeletal muscle; those with this disorder usually die in early 20s

Colorblindness Test

Sex-Linked Traits Y Xh XH Xh Females that are heterozygous for X-linked disorders are called Carriers: they don’t show the disorder but carry the defective gene Method to complete sex-linked Punnett squares is the same as before, except you use X and Y for the genotypes Ex: Cross a hemophilic male with a carrier female Genotypes: Phenotypes: Xh Y XH Xh

Sex-linked Punnett Genotypes: Phenotypes Cross a male with colorblindness with a normal female Genotypes: Phenotypes