Illustration of human chromosomes. Each of has 100 - 200 new mutations, but most have no noticeable effect.
Credit: iStockphoto
BRISBANE: You’re more like one of the X-men than you ever dreamed possible, says a new study. Every one of us has up to 200 new mutations – most of which are harmless – dotted throughout our DNA.
Using an advanced method of DNA sequencing, an international team of scientists have produced an estimate of the background rate of genetic mutation of human DNA, which is detailed today in the journal Current Biology.
“The amount of data we generated was unimaginable just a few years ago,” said study lead author, Yali Xue from the Wellcome Trust Sanger Institute in Cambridge, England. “But finding the tiny number of mutations was more difficult than finding an ant’s egg in the emperor’s rice store.”
10 million base pairs
The researchers compared a stretch of 10 million base pairs in the DNA of two Chinese men from the same village, whom records suggested were separated by 13 generations. They found only four new mutations.
From this they were able to extrapolate up to the whole genome and estimate how many of these mutations came from each generation. What they found suggests that each of has around 100 to 200 new mutations, which are tiny errors in the language of our DNA, that our parents did not have.
The results are remarkably similar to an estimate made in the 1930s by British geneticist JBS Haldane, who used data on male haemophiliacs to estimate that each of us has 150 new mutations.
For the study, the researchers took a stretch of DNA on the Y chromosome. This chromosome passes intact from father to son, and therefore undergoes far less change than the other chromosomes. This makes it an excellent candidate to study the background mutation rate.
Little change over time
The Chinese men used for the study last shared a common ancestor who was born in 1805. Despite all the generations of separation, the researchers only found 12 differences in the long stretch of DNA examined. Furthermore, of the 12, only four were true mutations, rather than mutations that had developed in lines of the mens’ cells cultured in the lab for the study.
“These four mutations gave us the exact mutation rate – one in 30 million nucleotides each generation – that we had expected,” said co-author Chris Tyler-Smith, also from The Wellcome Trust Sanger Institute.
He said that the findings may lead to new treatments for genetic diseases and offer new insight into the process of evolution.
“New mutations are responsible for an array of genetic diseases. The ability to reliably measure rates of DNA mutation means we can begin to ask how mutation rates vary between different regions of the genome and perhaps also between different individuals,” Tyler-Smith said.
Evolutionary geneticist Simon Easteal from the Australian National University, in Canberra, said that a reliable estimate of the mutation rate could be useful for helping us understand the factors which control the speed at which mutations develop – and would also allow us to determine how much time had passed since two people had last shared a common ancestor.
However, “there is some doubt about the number of generations that separate the two men in the study,” said Easteal, meaning the entire calculation could be wrong.
“If the estimation of the number of generations is inaccurate this would lead to an calculation of mutation rate which may be much less accurate,” he said.
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