What causes Fragile X Syndrome
Fragile X Syndrome (FXS) is caused by the expansion or lengthening of the FMR1 gene on the X chromosome, known as a gene mutation. The X chromosome is one of two sex determining chromosomes. When the gene lengthens it switches off production of a protein that is involved in brain development and other functions.
Females have two X chromosomes and males have one X and one Y. Normally, the X chromosome contains between 6 and 50 repeats of a gene (the FMR1 gene) responsible for producing a protein vital for normal brain development.
In some people, this gene may contain between 50 and 200 repeats.This is called a Fragile X premutation. A person with this premutation is called a ‘carrier’. FXS is caused when this gene lengthens to over 200 repeats interfering with the normal production of this protein and therefore normal brain development.
The Fragile X gene may be passed down through each generation. Daughters born to male carriers will inherit their fathers affected X chromosome and they will also be carriers. Sons born to male carriers will not inherit their fathers X chromosome and therefore will not be affected by Fragile X.
Because females have two X chromosomes, children of either sex born to female carriers have a 50% chance of inheriting the affected gene. If they inherit the affected gene they will either be a carrier or have full mutation Fragile X.
The Fragile X mutation often increases when passed from mother to child and down through the generations of a family. This means that children born with Fragile X frequently appear in families with no previous history of intellectual disability. Watch the digital animation below for a pictorial view of how the genetics of Fragile X work.
Click here to watch the 3D Animation of Fragile X and how its genetics work.