For most diploid species, sex is determined by one of the homologous pair of sex chromosomes.

In humans and fruit flies, the sex chromosomes are X and Y (XX=female ; XY=male). In birds, female is WZ and male is ZZ. There are many ways of determining sex:

  • Sex chromosome: humans, silkworms, drosophila, nematodes.
  • Ploidy: bees, wasps, and ants.
  • Environment: bonellia (sea worm larvae are undifferentiated; if lands on ocean floor, becomes female; if lands on female, becomes male).

Since in humans, Y chromosome carries very little genetic information, sex-linked genes usually refer to X-linked genes (genes on X chromosome). To determine if a gene is sex-linked, you perform pedigree analysis, use Drosophila, or perform a reciprocal cross.