An autopsy performed on one infant boy, who had a sister and an aunt with Rett, found that he also carried the mutant gene. But if you are a girl, you have two X chromosomes. You have a good copy and a bad copy. And the good copy could let you live, while the bad copy left you with this disease. All seems normal until suddenly their development goes into reverse.
Extremities become floppy.
The girls characteristically lose the use of their hands for all normal functions, but move them constantly, generally in a repetitious wringing action. Head growth begins to slow. Language and motor skills deteriorate. Most girls with Rett withdraw socially, acting much like an autistic child would. Many have acute epileptic-like seizures.