How many genes do I share with grass?

Harriet - Listener Martin Richards asks, "Given that all life on earth is supposed to be descended from a common source, how many genes do I share with a blade of grass? Here's Dr Aylwyn Scally from the Department of Genetics at the University of Cambridge...

Aylwyn - When we ask about shared genetic information between species, in other words, when we compared the DNA sequences or genomes of one species with another, we're really asking about the common ancestry of those species. So for example, with humans and chimpanzees, when we look at the chimpanzee genome, about 99 per cent of it matches up with some place in the human genome. And that's because chimpanzees are very close in evolutionary terms to us.

The common ancestor was about 6 million years ago - a long time for us, but short time for evolution. We go a bit further back to the common ancestor of say, humans and mice, that's about 10 times longer ago. There's been a lot more time for divergence and then we find only about 75 per cent. About 75 per cent of the mouse genome can be matched up almost exactly with some area in human. When it comes to comparing humans or any animal with a plant such as grasses, we're then talking about a much, much greater gulf in time, around about 1.5 billion years.

As you might expect, there's very little that remained unchanged in the genomes of either lineage since that time. Trying to make the same comparison that I just talked about, around about 25 per cent is representative thereabouts of the kind of similarity at that scale. However, we can relax the constraint somewhat. We look for similarity which is not exact, but nevertheless, close enough that there's similarity in function between regions of the genome. Then when we look at plant genes, we can find over half of them have some recognisable counterpart in humans. And that's presumably because those genes had an important role in the common ancestor and continue to do so in their descendants.

Kat - Thanks to listener Martin Richards, Cambridge University's Dr Aylwyn Scally and Harriet Johnson.