Autism-World

In 1991, scientists discovered the gene (called FMR1 for “Fragile X Mental Retardation – 1″) that causes fragile X. In people with Fragile X, a defect in FMR1 (a “full mutation”) shuts the gene down. Like a defective factory, FMR1 cannot manufacture the protein that it normally makes.Other individuals are carriers: they have a small defect in FMR1 (called a “premutation”) but do not show symptoms.

Fragile X is inherited. Carrier men pass the premutation to all their daughters but none of their sons. Each child of a carrier woman has a 50% chance of inheriting the gene defect. The Fragile X premutation can be passed silently down through generations in a family before a child is affected by the syndrome.

The FMR1 gene is located on the long arm of the X chromosome. At the start of this gene lies a region of DNA which varies in length from one person to another. Ordinarily, this stretch of DNA falls within a range of length that would be considered “normal”.

In some people, however, this stretch of DNA is somewhat longer; this gene change is called a “premutation”. Although a person who carries the premutation does not typically have symptoms of Fragile X, the stretch of DNA is prone to further expansion when it is passed from a woman to her children. When the stretch of DNA expands beyond a certain length, the gene is switched off and does not produce the protein that it is normally makes. This gene change is called a “full mutation”.

A male who inherits a full mutation exhibits Fragile X syndrome because his only X chromosome contains the mutated gene. A female may not be as severely affected because each cell of her body needs to use only one of its two X chromosomes and randomly inactivates the other.

This entry was posted on Wednesday, March 21st, 2007 at 10:09 pm and is filed under Fragile X Syndrome.