Question:

Males are affected by all the autism spectrum disorders more than females except for Rett syndrome. Rett syndrome is only seen in females and also it is rare. And throughout the world, only 1000 something cases are reported. Could you please tell me what is the reason for that?

Answer:

Rett syndrome is a much rarer condition than the spectrum of autism disorders. In the current classification system, the Diagnostic and Statistical Manual, 4th edition (DSM-IV), Rett syndrome is classified as one of the Pervasive Developmental Disorders along with other autism spectrum disorders.

 However, since the DSM-IV came out in 1994, scientists have identified the cause for Rett syndrome, a mutation in the MECP gene on the X chromosome. This same gene has NOT been associated with other autism spectrum disorders. It is thought that, when this occurs in females, they have some protection due to having two X chromosomes (one X chromosome affected by the mutation, the other X chromosome normal).
 
However, when it occurs in a male (who only has one X chromosome), the effects may be severe enough that it is not compatible with life. This would explain why we very rarely see a male with the MECP mutation.  

The fact that males have only one X chromosome and females have two, is also thought to be related to males outnumbering females with autism. The fact that females have an additional X chromosome, probably normal, that provides some benefit by reducing the effects of whatever it is that is the cause of the child’s autism.

Because the cause of Rett syndrome has been identified, and separates it from the other autism spectrum disorders, it will probably be re-classified in the next DSM and removed from the group of autism spectrum disorders.

For more information:

Go to the Autism health topic.