Shuffling the genetic deck

More recombination ‘hot spots’ detected in descendants of West Africans than of Europeans

Two new genetic maps of African-Americans reveal that people of West African descent have more hot spots where chromosomes mix and match genes than people of European heritage do.

Until recently scientists knew next to nothing about the process humans use to mix and match parents’ genes to create a unique combination in a child, says Chris Spencer, a population geneticist at the Wellcome Trust Centre for Human Genetics at Oxford University in England. This process, known as recombination, also helps chromosomes stick together until it is time to separate during egg and sperm production.

Previously, scientists could trace recombination only in families, and they thought that the exchange of genetic information happened at random. These new studies are the first to describe a way to use unrelated people to map genetic shuffling in populations. The studies also demonstrate that the genetic handover actually takes places at predetermined locations on chromosomes. “They make real contributions to methodology, and they really tell you something about biology,” says Spencer, who was not involved in either study.

People of West African ancestry have about 2,500 recombination hot spots that are inactive in people of European ancestry, David Reich of Harvard Medical School, Simon Myers of Oxford and large team of collaborators report online July 20 in Nature.

A variant of a protein called PRDM9 is responsible for creating the new recombination hot spots, the team shows. PRDM9 works a bit like a mail carrier delivering mail to certain ZIP codes: Where it stops along a chromosome, recombination can happen. The variant of the protein more common in people of West African descent can read more ZIP codes and stops at more addresses than the version of the protein usually found in Europeans.

An independently compiled map similarly found that West Africans have a higher recombination rate than people of European heritage. African-American and African-Caribbean people have an intermediate rate, researchers at UCLA and many collaborators report online July 20 in Nature Genetics.

The intermediate level is consistent with the idea that PRDM9 is in charge of recombination, says John Novembre, a population geneticist at UCLA and a coauthor of the Nature Genetics study. “It’s not that the recombination factors from one ancestry overwhelm the other,” he says. But the amount of a person’s West African or European ancestry does influence the recombination rate, the researchers found.

The team discovered five spots in the genome where recombination happens less often than expected. Four of these low points contained an inversion — a bit of chromosome that has been flipped the wrong way — that keeps the chromosomes from recombining. The team also found that recombination hot spots were sites on the chromosomes where additions or deletions of big chunks of DNA occur. Such additions and deletions can lead to diseases.

Both maps were created to improve studies aimed at finding genes that contribute to common diseases. The results indicate that maps for many more ethnic groups should be made, Novembre says.

The findings might also help explain why Africa has so much human genetic diversity. Researchers used to think it was because people who stayed in Africa never had their diversity squeezed down through genetic bottlenecks the way people whose ancestors migrated out of Africa did, Reich says. The new studies suggest that more recombination, due to the hot spots, may also have contributed to greater diversity, he says.

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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