An introductory guide to the genetics of Huntington’s disease.
How are alleles inherited? What are the chances of inheriting the gene?
Since the Huntington gene is not on a sex-determining chromosome, the disease is not sex-linked. In other words, the inheritance and development of Huntington’s disease are not related to an individual’s sex. This means that males and females have an equal chance of inheriting the disease. Males and females with the disease are also equally likely to pass it on to their children.
Every person inherits two copies of the Huntington gene, one from each parent. Likewise, every person will also pass one of these two copies to each child. The chances of giving either of these two alleles to a child are equal (50/50). A person with Huntington’s disease has one non-HD allele and one HD allele. Hence, there is a 50% chance that the non-HD allele will be passed on and a 50% chance that the HD allele will be passed on. This means that each child of an individual with HD has a 50% chance of getting the HD allele. Individuals with a chance of inheriting the disease are sometimes described as “at-risk.” At-risk individuals have the option of undergoing genetic testing, which shows whether their "50% risk" of developing HD is in reality nearly 0% or nearly 100%. (For more information about genetic testing for HD, click here.)
Individuals without any copies of the HD allele do not have HD, and these individuals are very unlikely to pass HD on to their children. They have two non-HD alleles, and the child will always receive one of these two alleles. The only exception is in the case of a new mutation, a heritable change in a person’s DNA. Very rarely a mutation will occur so that a child’s allele differs from that of the parent from whom it was inherited. Only very rarely, therefore, does an individual without an HD allele have a child with HD. For the same reason, Huntington’s disease does not typically “skip” generations. That is, we do not observe families in which a grandparent and grandchild have HD but the child’s parents do not. If such a pattern were observed, it would be most likely that one of the parents has an HD allele, but has not yet developed symptoms of the disease.
Last Modified: 9-13-02
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