The higher proportion of such harmful mutations in European-Americans suggests that natural selection was more effective in African populations at getting rid of many of the mutations, Bustamante's team said.
In Europe, where population growth is historically more recent, there has been less time for natural selection to kick out a lot of these damaging mutations.
(Read related story: "Aborigines, Europeans Share African Roots, DNA Suggests" [May 7, 2007].)
Joshua Akey is a genome scientist at the University of Washington, Seattle. He said the study is significant because it shows that the demographic history of European populations has caused a type of natural selection referred to as "purifying selection" to be less effective in Europeans.
Purifying selection, he explained, acts like a sieve to filter out harmful genetic mutations.
Bustamante's findings suggest that Europeans harbor more harmful mutations because of their more recent growth in numbers, he explained.
Akey thinks the findings may also have direct implications on human health.
"Perhaps these deleterious [mutations] contribute to the burden of complex and common diseases such as diabetes, asthma, and heart disease," he said.
In a related study also appearing today in Nature, a second group of researchers announced that they have produced genetic data that is a hundred times more detailed than previous global assessments of human populations.
Using more than 500,000 DNA markers from 485 volunteers from the Human Genome Diversity Project, the researchers analyzed the scope of genetic variations in 29 different populations across the world.
The exhaustive approach provides researchers with a much clearer picture of human population relationships and migrations, the team said.
"Previously you could tell from a genetic perspective whether someone was from Africa or which part of their genome was from where," said Andrew Singleton, a genetics researcher at the National Institutes of Health.
"This allows us to do that on a much finer scale and find which region someone is from."
The researchers also found 507 previously unknown bits of DNA, called copy-number variants, that could potentially help speed the discovery of disease-related genes.
"We used to think of the genome as a string of letters three billion base pairs long, but what's become quite evident is that each of us has extra bits, and bits missing," Singleton explained.
While the function of such chunks of DNA is not fully clear, they may be potential markers of disease.
"There are copy-number variants we think are completely benign, and there are those that can cause rare diseases. What is not known is the role they play in susceptibility to disease," he added.
For now the researchers plan to catalog the variants from different populations into a public database that could someday help match a certain variant to, say, a rare genetic disorder.
"There are really two main things we're trying to achieve. We want to get a handle on genetic diversity across different populations, as well as create a catalogue of genetic diversity as a starting point in understanding genetics and disease in different populations," Singleton said.
John Storey is a genomics researcher at Princeton University. He found "the strong relationships between genetic variation and geographic distance at this level of detail [in the study] to be particularly remarkable."
Storey said the findings could help better understand how a gene-disease association found in a particular study applies to human populations all over the world.
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