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15 - Ethics and the Gene Map page 7
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7. Huntington's disease Link to the Medical Research Council web site
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The inheritance of Huntington's disease
Huntington's disease is caused by an autosomal dominant mutation on chromosome-4 which is found in about 1 in 10,000 people. It usually has no phentypic effect until the person is aged between 25 and 55 and so is called a late-onset disorder. Eventually, though, it leads to a progressive deterioration of the nervous system. Affected people have problems with their balance and find motor co-ordination increasingly difficult. Death usually occurs within five to ten years of the disease's onset.

The mutation that causes Huntington's disease is due to the presence of a number of repeat CAG codons in the gene. If you have 34 or fewer of these repeat codons, you won't get Huntington's disease. If you have more than 42 you will eventually get the disease, and the more repeat codons you have, the younger you are likely to be when you get the disease. If you have between 35 and 42 repeat codons you may or may not ever develop the disease.

Question 4

Suppose a couple, one of whom has the allele for Huntington's disease, has a single child. What is the probability of that child developing the condition?