11 february 2005

Researchers find surprising difference between human and chimp genomes

Despite sharing 99 per cent of our DNA with chimpanzees, a certain key genetic process tends to occur at totally different places on human and chimp chromosomes. A study by Oxford statisticians and US and Dutch geneticists, published in Science, compared recombination in humans and chimpanzees and found a surprising difference between the species.

Recombination is the process by which genetic information inherited from a person's two parents is mixed up to make one chromosome to pass on to their offspring. Chromosomes exist in pairs, with one chromosome of the pair inherited from the father, the other from the mother. At the point when sperm or eggs are made, the paired chromosomes line up and exchange pieces of DNA, recombining into a totally new, single chromosome, which is passed on to offspring.

In a previous study in Science the Oxford team had identified many 'hotspots' along the human genome where this swapping of DNA is more likely to occur. This has important consequences for our understanding of human genetic variation, and for the way in which scientists should look for genes likely to play a role in disease.

Why these hotspots occur, and what triggers the swapping of DNA at those particular points, is a mystery. One theory was that the DNA code either side of hotspots controlled the activity. However, comparing chimps and humans showed that despite being so genetically similar, the species have totally different recombination hotspots.

Dr Simon Myers, from the Department of Statistics, developed the computational and statistical methods which allow the identification of recombination hotspots from patterns of genetic variation in samples of individuals from the relevant populations, building on earlier work by his colleague Dr Gil McVean. Professor Peter Donnelly, who jointly led the study with Professor David Altshuler of the Broad Genome Institute, said: 'If chimps and humans do not share these recombination hotspots, then it means something other than the surrounding DNA code must be controlling the process of recombination - because the surrounding DNA code in chimps and humans is pretty much identical. This means that recombination is even more mysterious than we already thought: what is controlling it, and why does it occur so often at these particular places?

'The findings also tell us something else important: that the recombination landscape must be evolving extremely quickly. In humans and chimpanzees, the genome as a whole is very similar but the recombination hotspots totally different - so hotspots must be evolving much, much faster than the rest of the genome. That adds extra mystery to what drives these hotspots: why do they evolve so quickly?'

As well as showing the difference between human and chimpanzee recombination, the paper represents the first time that recombination hotspots have been identified in the chimpanzee genome.