Sex-linked inheritance. Gene interaction

Traits controlled by genes on the sex chromosomes are inherited linked to sex. The X chromosome is large and carries many genes unrelated to sex; the Y chromosome is small and has no matching allele for most genes of the X. So in a male a recessive gene on the X always shows in the phenotype (a hemizygous state). Colour blindness (inability to distinguish colours) and haemophilia (a blood clotting disorder) are X-linked recessive traits (XᴴXʰ – a carrier woman, healthy but able to pass the allele on). A woman is affected only if both X chromosomes carry the recessive allele (XʰXʰ), which is why these conditions are commoner in men. A father passes an X-linked trait only to his daughters, and gives his sons the Y; a son gets his X from his mother. Carrier mother (XᴴXʰ) × healthy father (XᴴY): all daughters are healthy (half are carriers), and half of the sons are affected. Y-linked traits pass only along the male line. T. Morgan discovered this pattern in Drosophila using eye colour (red is dominant, white recessive, the gene lies on the X chromosome). In birds the female is heterogametic (ZW), so a sex-linked trait (for example barred plumage in chickens) shows in females with a single allele.