Why does Rett syndrome only occur in girls?

Rett syndrome is not only found in girls. However, it is overwhelmingly diagnosed in girls, with a ratio of about 1 in 10,000 female births compared to 1 in 500,000 male births. The reason for this gender bias is related to the genetics of the condition.

Rett syndrome is caused by mutations in the MECP2 gene, which is located on the X chromosome. Females have two X chromosomes, while males have one X chromosome and one Y chromosome. This means that females have a "backup" copy of the MECP2 gene in case one copy is mutated. Males, on the other hand, do not have this backup, so a single mutation in the MECP2 gene can lead to Rett syndrome.

In addition to the gender bias, Rett syndrome also shows variable expressivity, meaning that the severity of the symptoms can vary widely even among individuals with the same mutation. This variability is thought to be due to a combination of genetic and environmental factors.