We are more unique than previously thought, according to new comparisons of human and chimpanzee DNA.
It has long been held that we share 98.5 per cent of our genetic material with our closest relatives. That now appears to be wrong. In fact, we share less than 95 per cent of our genetic material, a three-fold increase in the variation between us and chimps.
The new value came to light when Roy Britten of the California Institute of Technology became suspicious about the 98.5 per cent figure. Ironically, that number was originally derived from a technique that Britten himself developed decades ago at Caltech with colleague Dave Kohne. By measuring the temperature at which matching DNA of two species comes apart, you can work out how different they are.
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But the technique only picks up a particular type of variation, called a single base substitution. These occur whenever a single 鈥渓etter鈥� differs in corresponding strands of DNA from the two species.
But there are two other major types of variation that the previous analyses ignored. 鈥淚nsertions鈥� occur whenever a whole section of DNA appears in one species but not in the corresponding strand of the other. Likewise, 鈥渄eletions鈥� mean that a piece of DNA is missing from one species.
Littered with indels
Together, they are termed 鈥渋ndels鈥�, and Britten seized his chance to evaluate the true variation between the two species when stretches of chimp DNA were recently published on the internet by teams from the Baylor College of Medicine in Houston, Texas, and from the University of Oklahoma.
When Britten compared five stretches of chimp DNA with the corresponding pieces of human DNA, he found that single base substitutions accounted for a difference of 1.4 per cent, very close to the expected figure.
But he also found that the DNA of both species was littered with indels. His comparisons revealed that they add around another 4.0 per cent to the genetic differences.
Junk and genes
鈥淲e鈥檙e not any more different than we were,鈥� says Britten. 鈥淏ut we see a bit more divergence than before because insertions and deletions are taken into account. It almost triples the difference.鈥�
The result is only based on about one million DNA bases out of the three billion which make up the human and chimp genomes, says Britten. 鈥淚t鈥檚 just a glance,鈥� he says.
But the differences were equally split between 鈥渏unk鈥� regions that do not have any genes, and gene-rich parts of the genome, suggesting they may be evenly distributed.
Britten thinks it will be some time before we know what it is about our genes that makes us so different from chimps. He thinks the real secrets could lie in 鈥渞egulatory鈥� regions of DNA that control whole networks of genes. 鈥淚t鈥檒l be a while before we understand them,鈥� he says.
Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.172510699)
