Fragile X Syndrome (cont.)
How is Fragile X syndrome inherited?
The gene for Fragile X is carried on the X chromosome. Because both males (XY) and females (XX) have at least one X chromosome, both can pass on the mutated gene to their children
| Diagram A |
|
|
| |
Father |
|
| |
Y |
XFragile |
Mother |
|
|
| X |
Boy
XY |
Girl
XXFragile |
| X |
Boy
XY |
Girl
XXFragile |
A father with the altered gene for Fragile X on his X chromosome will only
pass that gene on to his daughters. He passes a Y chromosome on to his sons,
which doesn't transmit the condition. Therefore, if the father has the altered
gene on his X chromosome, but the mother's X chromosomes are normal, all of the
couple's daughters would have the altered gene for Fragile X, while none of
their sons would have the mutated gene (see Diagram A).
Current research indicates that a father can pass on the premutation form of
the FMR1 gene only to his daughters. In other words, if a daughter inherits the
mutated FMR1 gene from her father, she will get only the premutation from him,
not the full mutation. Even if the father himself has a full mutation, it
appears that sperm can carry only the premutation. Scientists don't understand
how or why fathers can only pass on the milder form of Fragile X to their
daughters. This remains an area of focused research.
Because mothers pass on only X chromosomes to their children, if the mother
has the altered gene for Fragile X, she can pass that gene to either her sons or
her daughters. If the mother has the mutated gene on one X chromosome and has
one normal X chromosome, and the father has no genetic mutations, all the
children have a 50-50 chance of inheriting the mutated gene (see Diagram B).
| Diagram B |
|
|
| |
Father |
|
| |
Y |
X |
Mother |
|
|
| XFragile |
Boy
YXFragile |
Girl
XXFragile |
| X |
Boy
XY |
Girl
XX |
The odds noted here apply to each child the parents have. Having one child
who receives an X chromosome with the FMR1 mutation does not increase or
decrease the chances of having another child with the mutated FMR1 gene. Nor do
these odds influence the severity of the symptoms. Having one child with mild
symptoms does not mean that the other children will have severe symptoms, and
having a child with severe symptoms does not mean that other children will have
mild symptoms.
A premutation gene is less stable than a full mutation. In some cases, the
mutated gene may expand from the premutation to the full mutation as it is
passed on from mother to child. The chances of expansion depend on the number of
repeats in the promoter of the premutation gene; the higher the number of
repeats, the more likely it is that the gene will expand. These chances also
increase with each generation. Children of a mother who has the premutation,
then, may have no genetic mutation, the premutation, or the full mutation.
Further, because an altered FMR1 gene can be passed on without symptoms, many
people are unaware that they have it. As a result, a premutation form of the
FMR1 gene can be silently passed through a family for generations, with no one
ever showing any symptoms. However, with each generation, it becomes more likely
that the premutation gene will expand its number of repeats to become a full
mutation gene, which would also increase the number of and seriousness of
symptoms.
Next: What are the signs and symptoms of Fragile X syndrome? »
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