Some Traits (Including Some Diseases) Depend on Many Genes Rather than a Single Gene

(p. D3) A new exploration of how evolution works at the genomic level may have a significant impact on drug development and other areas of medicine.
The report, published in Nature last week, offers new evidence in a longstanding debate about how organisms evolve. One well-known path to change is a heavily favorable mutation in a single gene. But it may be well known only because it is easy to study. Another path is exploitation of mildly favorable differences that already exist in many genes.
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Three biologists at the University of California, Irvine, Molly K. Burke, Michael R. Rose and Anthony D. Long, followed populations of fruit flies through 600 generations and studied the whole genome of some 250 flies in order to see what kinds of genetic change they had undergone.
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The conventional view is that evolutionary change is generally mediated by a favorable mutation in a gene that then washes through the whole population, a process called a hard sweep because all other versions of the gene are brushed away. The alternative, called a soft sweep, is that many genes influence a trait, in this case the rate of maturation, and that the growth-accelerating versions of each of these genes become just a little more common. Each fly has a greater chance of inheriting these growth-promoting versions and so will mature faster.
In sequencing their subjects’ genomes, the researchers found that a soft sweep was indeed responsible for the earlier hatching. No single gene had swept through the population to effect the change; rather, the alternative versions of a large number of genes had become slightly more common.
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Haldane favored the single mutation mechanism, but Fisher and Wright backed multiple gene change.
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The demise of the Haldane view “is very bad news for the pharmaceutical industry in general,” Dr. Rose said. If disease and other traits are controlled by many genes, it will be hard to find effective drugs; a single target would have been much simpler.

For the full story, see:
NICHOLAS WADE. “Natural Selection Cuts Broad Swath Through Fruit Fly Genome.” The New York Times (Tues., September 21, 2010): D3.
(Note: ellipses added.)
(Note: the online version of the article is dated September 20, 2010.)

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