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Q&A MedQuest: Inheritance of the Marfan Syndrome
How does someone get the Marfan syndrome?
In 65-75 percent of cases, the Marfan syndrome is inherited from a parent. Its pattern of inheritance is called autosomal dominance.
Autosomal refers to the fact that the gene causing the disorder does not occur on one of the sex chromosomes. Therefore, the sex of the affected parent and the sex of the child have nothing to do with who is affected. Dominant refers to the fact that only one of the two copies of the causative gene needs to carry the mutation for the disorder to be present.
When one parent has the Marfan syndrome, each child born has a 50-50 chance of being affected by the disorder, regardless of the sex of the child or the affected parent.
For the one-quarter to one-third of people with the Marfan syndrome who do not have an affected parent, a spontaneous mutation occurs in the egg or the sperm of a parent in the gene that causes the Marfan syndrome. On average in such families, the age of the father is older than that of fathers in the general population, suggesting that the spontaneous mutation occurs most often in the cells of the testes, the male reproductive organ that produces sperm. Mutations that cause the Marfan syndrome arise quite infrequently, about once in every 20,000 births.
When the Marfan syndrome occurs in a family because of a new mutation, is the gene lurking somewhere in the family’s past? Does the Marfan syndrome skip generations?
In virtually all cases, someone who has one copy of the mutant gene for Marfan syndrome shows some outward sign of the condition. This is unlike recessive forms of inheritance, in which a parent can be a "carrier" of a mutation and be unaffected. If a relative of someone with the Marfan syndrome shows no signs of the condition, then the relative is extremely unlikely to have inherited the mutant gene and, therefore, equally unlikely to have a child with the Marfan syndrome. Therefore, for purposes of genetic counseling, Marfan syndrome does not skip generations.
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