Differences in the error mechanisms affecting sex and autosomal chromosomes in women of different ages within the reproductive age group.

Aneuploidy was assessed in lymphocyte cultures from 16 women aged between 20 and 50. Between 236 and 1,677 cells were studied per subject and the gains and losses of each chromosome recorded. The X chromosome was found to show the same ratio of loss to gain (approximately 3:1) at all ages, but the frequency of total aneuploidy (loss plus gain) showed a significant increase with age. By contrast, there was no clear association of the frequency of autosomal chromosome aneuploidy with age, but the ratio of loss to gain was significantly greater in younger women (aged 21-35) than in older women (aged 36-50). Thus, X chromosomes in females are affected by a mechanism of error different to that affecting autosomal chromosomes. Although the ratio of loss to gain changes, the relative involvement of the different autosomal chromosomes is unchanged with age. Thus, the initial "recruitment" of chromosomes undergoing error is the same in both groups, but the "processing" of these chromosomes is different. Since the relative involvement of autosomes in aneuploidy mimics their relative involvement in displacement, it is proposed that displacement is the initial or "recruitment" step in error. "Processing" then commonly involves "chromosome elimination" in younger women and more frequent "random segregation" in older women.