It's also unclear if the acceleration seen in the chimp and human ancestor is unique. "These basic kinds of mutations have been going on for at least 90 million years," says Nick Patterson, a geneticist at the Broad Institute, in Cambridge, MA. "The question is whether there is something unusual in what happened in human lineage; I doubt we have enough data to answer that." This type of comparison would require genome sequences for many related mammal species. Duplications are likely to have very different evolutionary properties than single-letter changes. Both arise from mistakes at the molecular level, which can then either help, harm, or do nothing to the reproductive fitness of the organism. Most single-letter changes fall into the neutral category. But because duplicative changes often increase the number of copies of a gene and thus potentially increase the concentration of protein that gene produces, they are more likely to exert an effect on the carrier. In addition, while single-letter changes may make a particular protein more or less effective by slightly tweaking its structure, duplications that create additional copies of specific genes free up the new copies to evolve an entirely new purpose. "You havetwo copies that can diverge from each other," says Perry. "One copy can then experience mutation and attain a new function that could be important for the biology of that organism." For example, color vision in primates arose thanks to the duplication of the gene for visual pigment. "With this kind of analysis," says Perry, "we can begin to identify other genes specific to different lineages, and then study the potential effect they might have on the biology of these species." Most of the duplications analyzed in the study--more than 80 percent--are shared by humans, chimps, and gorillas. But the genes in duplicated regions unique to humans are largely ones that have not yet been characterized. "We found more than 30 genes that are duplicated only in humans," says Marques-Bonet. "But we still don't know what they do." |
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The Holes in Our Genomes
09/19/2008
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Massachusetts Institute of Technology
Tags
copy number variation DNA DNA deletion evolution gene expression genetic diversity genome primate