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.
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).
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.
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