Huntington's Chorea: Evolution and Genetic Disease
How is it possible that such a devastating genetic disease is so common in some populations? Shouldn't natural selection remove genetic defects from human populations? Research on the evolutionary genetics of this disease suggests that there are two main reasons for the persistence of Huntington's in human populations: mutation coupled with weak selection.
The diagram at left shows how the Huntington's allele is passed down. Since it is the dominant allele, individuals with just one parent with Huntingtons's chorea have a 50-50 chance of developing the disease themselves.
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.
Today, genetic testing can identify people who carry a Huntington's allele long before the onset of the disease and before they have made their reproductive choices. The genetic test that identifies the Huntington's allele works sort of like DNA fingerprinting. A DNA sample is copied and cut into pieces. The pieces are then spread out on a gel (see right). The banding pattern can tell researchers whether a person carries an allele that is likely to cause Huntington's.
Having this information could allow people to make more-informed reproductive decisions. For example, at Lake Maracaibo, researchers and health workers have tried to make contraception available to the local population so that they can make reproductive choices based on their own family history with the disease. But whatever people eventually decide to do with this knowledge, a deep understanding of the disease would not be possible without the historical perspective offered by evolution.
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1A Tale of Pain and Hope on Lake Maracaibo (int'l edition), Business Week Online. May 29, 2000.
Photo of Venezuelan familiy © 1983 by Steve Uzzell; Dr. Wexler photo © 1986 by Steve Uzzell
Understanding Evolution © 2015 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California