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Single Gene Disorders and Disability (SGDD) |
The FMR1 gene is on the X chromosome. Like all chromosomes, the X
chromosome is passed from parent to child. Females have two X
chromosomes, and males have one X chromosome and one Y chromosome.
To see how X and Y chromosomes pass from parents to children, see
About X-linked conditions.
People with a normal number of repeats
In addition to being X-linked, the inheritance pattern of
FXS in families is affected by the number of CGG repeats in the
gene. Most people have about 6 to 45 repeats in their FMR1
genes. Repeats of this size are stable in families. That is, the
number of repeats does not change when passed from parent to
child.
People with an intermediate number of repeats
People who have an intermediate repeat size (about 45 to 55)
are not at risk for having children with FXS. However, when the
gene is passed on to their children, the number of repeats can
grow to a premutation size. Children who have a premutation are
then at risk for having children with FXS. Therefore, a person
with an intermediate size repeat is at risk for having
grandchildren with FXS.
People with a premutation
A premutation repeat (about 55 to 200 repeats) can grow to a
full mutation when the gene is passed from a mother to a child.
Therefore, a woman with a premutation is at risk for having a
child with FXS. The more repeats she has, the more likely it is
to grow to a full mutation when passed to her children. On the
other hand, in men, the premutation does not grow, so men with
premutations do not pass on a full mutation. A man with a
premutation will pass the premutation to his daughters, because
he passes on his X chromosome to all his daughters. Sons of men
with premutations will not be affected, because boys get a Y
chromosome from their fathers instead of an X chromosome.
People with a full mutation
If men with FXS (over 200 repeats) have children, they do
not pass on the full mutation. Rather, the full mutation shrinks
back to a premutation size and their daughters will have
premutations. The sons of males with FXS will not be affected,
because boys get a Y chromosome from their fathers instead of an
X chromosome.
Females who have a full mutation have a 50% chance with each
pregnancy of passing on the full mutation to her children.
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Date: August 28, 2006
Content source: National Center on Birth Defects and Developmental
Disabilities
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