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
