17. Huntington\'s Disease (HD) is a devastating, degenerative brain disorder for

Question

17. Huntington's Disease (HD) is a devastating, degenerative brain disorder for which there is, at present, no effective treatment or cure. Symptoms usually appear in adults within the third or fourth decade of life and result in death within 10 to 20 years. HD is an autosomal dominant disease. A young patient, whose paternal grandfather died of HD (grandmother lived to be 102!!!) and whose parents (age 27) are as yet healthy would like to know whether she should be worried. Her maternal grandparents are both healthy and free of HD. What would you advise her concerning the probability of her having HD? One out of every 10,000 Americans has HD. this lethal autosomal dominant allele? Can you explain why selection has not removed

Explanation / Answer

Since it is the dominant allele, individuals with just one parent with Huntingtons's have a 50-50 chance of developing the disease themselves.

In 1993, a collaborative research group discovered the culprit responsible for Huntington's: a stretch of DNA that repeats itself over and over again, CAGCAGCAGCAG... and so on. People carrying too many CAGs in the Huntington's gene (more than about 35 repeats) develop the disease. In most cases, those affected by Huntington's inherited a disease-causing allele from a parent. Others may have no family history of the disease, but may have new mutations which cause Huntington's.

If a mutation ends up inserting extra CAGs into the Huntington's gene, new Huntington's alleles may be created. Of course it's also possible for a mutation to remove CAGs. But research suggests that for Huntington's, mutation is biased; additions of CAGs are more likely than losses of CAGs.

Selection
As though that weren't bad enough, Huntington's belongs to a class of genetic diseases that largely escape natural selection. Huntington's is often "invisible" to natural selection for a very simple reason: it generally does not affect people until after they've reproduced. In this way, the alleles for late-onset Huntington's may evade natural selection, "sneaking" into the next generation, despite its deleterious effects. Early-onset cases of Huntington's are rare; these are an exception, and are strongly selected against.

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