Linked inheritance and sex genetics

Chromosomes are of two kinds: autosomes (the same in male and female) and sex chromosomes (X and Y; they differ between the sexes). A human has 46 chromosomes = 22 pairs of autosomes + 2 sex chromosomes: a woman is XX (the homogametic sex, every egg has X), a man is XY (the heterogametic sex, half of the sperm have X and half Y); sex is fixed at fertilisation depending on whether the sperm is X or Y, so the sex ratio is about 1:1. In birds, butterflies and many reptiles it is the other way round — the female is heterogametic (ZW) and the male ZZ; in some reptiles sex also depends on incubation temperature. Inheritance of genes on the X chromosome is sex-linked: the white-eye gene (w) in Drosophila is recessive; in humans haemophilia (a blood-clotting disorder) and colour blindness (not telling red from green) are recessive X-linked genes, which show up more often in males because they have no second X. Genes on the Y chromosome pass only from father to son. Example: a woman carrying the haemophilia gene (XᴴXʰ) × a healthy man (XᴴY): all daughters are healthy (half are carriers), and half the sons are affected (XʰY).