ABSTRACT
These differences in male and female gametogenesis can have important consequences; as a woman ages, her primary oocytes age also. This means that genetic defects are more common in the ova of older women (from their mid-30s onwards), which is one of the reasons why the incidence of genetic abnormalities rises as women get older. For example, Down's syndrome is a genetic disease in which children have three copies of chromosome 21 (trisomy 21), rather than the usual two copies, and the rate of birth of children with Down's syndrome in women aged 40 years (1:100) is ten times greater than that of children born to women aged 29 years (1:1000). This is caused largely by the fact that primary oocytes in older women are themselves 'older', so that more errors are made during the second stage of meiosis.
