Approximately 0.1% of females have an extra X chromosome. It is likely that there will be no clinical features, however, some studies suggest that:
- intelligence is typically reduced by 10-20 IQ points. Special educational requirements are rare
- they may show aggressive / unsociable behaviour
95% of cases are due to problems in maternal meiosis. Most patients are fertile, and will not usuallypass on the defect
Extra X chromosomes
Further X chromosomes (e.g. XXXX, XXXXX) are associated with a reduced intellectual capacity proportional to the number of X chromosomes present.
The same is true for males with extra X chromosomes, such as XXY, XXXY.