An introductory guide to the genetics of Huntington’s disease.
Are my children at risk?
Let’s switch gears and think about this question from the perspective of the child of a person with HD. The child inherits one allele from each parent. The parent without HD has two non-HD alleles, so the allele from this parent will be non-HD regardless of which one is inherited. The parent with HD has one non-HD allele and HD allele. There is an equal probability of passing either of these alleles to the child. Thus the child has a 50% chance of getting the non-HD allele and a 50% chance of getting the HD allele. Since the chance of getting an HD allele is one in two, the child has a 50% chance overall of inheriting the disease. (See Figure C-2.)
What if you discover that you are the grandchild of a person with HD, and your parent who is at risk chooses not to be tested? Recall that you have two copies of the Huntington gene. One copy (allele) will come from the parent who is not at risk. This copy will always be non-HD and does not affect your chances of getting the disease. The second copy comes from your at-risk parent. Since this parent is the child of an individual with HD, he or she has an equal chance of having either two non-HD alleles or one non-HD and one HD allele. (We found this out in Figure C-2.) In the first case, this parent has two non-HD alleles and you will not inherit the disease regardless of which of the two non-HD alleles you get. In the second case, the parent has one HD allele and one non-HD allele. Here, you will inherit the disease if you get the HD allele, but not if you get the non-HD allele. Out of the four possible outcomes, exactly one results in your having a copy of the HD allele. This represents a 25% chance that you have inherited the disease. (See Figure C-3.)
Last Modified: 9-13-02
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