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The X Chromosome


Picture of how our X and Y chromosomes are inherited Our genes are located on structures called chromosomes. Most people have 46 chromosomes (23 pairs). Our gender is determined by the X and Y chromosomes: males have one Y chromosome and one X chromosome, while females have two X chromosomes.


What happens when a female is a carrier of fragile X syndrome?

As females have two X chromosomes, each child has a 50% chance of receiving either one or the other X chromosome from their mother. So if a woman has one X chromosome with the FMR1 gene in the normal range and one X chromosome with the FMR1 gene in the carrier range she can pass on either version to her children.

If the child receives the X chromosome with the FMR1 gene in the carrier range, the size of the gene may increase to the long length gene and the child will be affected by fragile X syndrome.


What happens when a male is a carrier of fragile X syndrome?

If a male passes on his Y chromosome he will have a son and if he passes on his X chromosome he will have a daughter. As the FMR1 gene is on the X chromosome, males can only pass on their FMR1 gene to their daughters.

The length of the FMR1 gene does not increase in size when passed from father to child. So males who have an FMR1 gene in the carrier range will pass it on to all of their daughters but the length of the gene won’t change.


Why are females less affected by fragile X syndrome than males?

Females with fragile X syndrome tend not to be as severely affected as males with fragile X syndrome. This is because females have two X chromosomes and the second X chromosome with a FMR1 gene in the normal range can usually compensate for the X chromosome with a long FMR1 gene.


Where does the name “fragile X syndrome” come from?

Where does the name “fragile X syndrome” come from?

The name fragile X syndrome is derived from its link to the X chromosome. This is because under certain laboratory conditions the tip of the X chromosome can appear broken or fragile in people with fragile X syndrome.