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Why so many rare variants?

chromosomesSM.jpgUnsurprisingly, this year much of the buzz at the American Society of Human Genetics meeting is about the 1000 Genomes Project, the eagerly awaited pilot data for which was published last week. One of the main take-home messages from the analysis has been the huge number of rare variants the study has turned up so far.

In a talk earlier today here in Washington DC, Andy Clark, a population geneticist at Cornell University, gave a tantalizing explanation for why rare variants are so numerous: population size. For millennia, population growth had been fairly static, but over the last couple centuries it has begun to shoot up wildly, with the growth rate now at about 1-2% per year.

That expansion, he says, “grossly distorts our geneology.” With so many people, there hasn’t been time for natural selection to weed out all those deleterious variants.

“We’re simply a mutation accumulation experiment that’s been going on for centuries now,” Clark said in the talk. “The explosive population growth actually results in the growth of deleterious mutations on the individual as well.”

Image: Human karyotype; NIH via Wikimedia Commons

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