Most genetic changes in humans occurred in past 5000 years, finds new study
NEW DELHI: In the past 5000 years, the human genome - the genetic code carried in our DNA - has accumulated a large number of variations, many of them potentially harmful. This has happened because of exploding human population, which causes naturally arising genetic mutations (changes) to keep collecting and getting passed on to progeny.
A study published today in the scientific journal Naturehas detailed when many of those rare variants arose. The study had a fairly large sample of 4,298 North Americans of European descent and 2,217 African Americans which enabled the researchers to gather unprecedented details from the genetic code. Study co-author Josh Akey, a genomics expert at the University of Washington in Seattle told Nature that the researchers now have "a way to look at recent human history in a way that we couldn't before."
Researchers identified the origins of more than 1.15 million single-nucleotide variants, that is, "locations where a single letter of the DNA sequence is different from other individuals" in the genomes of the studied sample. These changes were found in those sections of the genes that provide the template for building proteins. Besides confirming that these variations occurred during the past 5,000-10,000 years, they also found that changes reflected different routes and times of migration by humans.
Of all the changes, nearly three-fourths - 73 percent - arose the past 5,000 years, the researchers report in Nature. About 14 percent of the variants were potentially harmful, and of those, 86 percent arose in the past 5,000 years.
It is these variants that could be contributing to vulnerability or inclination to specific diseases. "There's so many of variants that exist that some of them have to contribute to disease," says Akey, quoted in Nature.
Does that mean that humans today are more susceptible to diseases? Not necessarily, say the researchers. What it suggests is that most diseases are caused by more than one variant, and also, that diseases could operate through different genetic pathways and mechanisms in different people. It was earlier believed that common diseases are caused by common variations. This led to a popular perception that diseases could be cured by simply identifying the variations causing them. This study debunks that belief. It is the rarer variations that may be responsible for susceptibility to diseases.
The researchers found that the European Americans had a larger proportion of potentially harmful variants. This was probably because of what is called the 'genetic bottleneck' that happened when they originally migrated out of Africa. The first small group of humans that left Africa for Europe would have been forced to mate amongst themselves for some time. This would have caused a sudden drop in genetic diversity or a 'bottleneck'. In the rapid expansion in population size that followed, selection was slow to catch up to and weed out potentially harmful mutations, Nature reported.
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