Huntington’s Chorea: Evolution and Genetic Disease (2 of 3)

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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.

Replication of CAGs

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.

Read more on the genetics of Huntington’s Disease at Stanford University’s HOPES site.

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