Sex Determination

Human sex determination occurs according to the X - Y system

  • Females have two copies of the larger X chromosome
  • Males have one X and one Y chromosome (and hence determine gender in offspring)

Other Systems

X - 0 System

  • Common in certain insects, including grasshoppers and crickets
  • Females have two copies of the X chromosome (XX), whereas males only have one chromosome (X0)
  • Generally, in this method, sex is determined by the overall level of gene expression by the X chromosome (less in males)

Z - W System

  • Common in birds, some reptiles and certain insects
  • Males have two copies of the Z chromosome (ZZ), whereas females have one Z and one W chromosome (ZW)
  • Certain essential female genes appear to reside on the W chromosome (similar to the Y chromosome in human males)

Haplo-diploid System

  • Common in certain insect species, such as ants and bees
  • Unfertilised eggs develop into haploid individuals, which are males
  • Fertilised eggs develop into diploid individuals, which are generally female (all bar the reproductive queen are sterile)

Sex Determination Systems

sex determination systems