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Steve Jones: Well inbreeding really is genetics. What inbreeding is, it tells you that you have copies of your genes in common from a shared recent ancestor. Perhaps you married your cousin which means you’ve got a grandparent in common so your children will be inbred. Now, what we’ve got in the human genome sequence as here is really a very primitive approach to it, it’s just one copy of the human genome from one person. The next step, which has already been done, is for somebody to take their genome twice, like you’ve got two copies of your genome, one that came from your father and one that came from your mother, so the next generation of these will have two lines where there’s only one here. And what you can do is read along the lines and if there are great segments which are identical to each other than that’s a pretty good statement that those identical segments come from your shared grandparent and hence that you’re inbred.

So that looking at genes is really a very good way of actually measuring inbreeding. And you could do that across the world. You could look at particular populations in Pakistan, let’s say, which are pretty inbred, or populations on oceanic islands where nobody’s come in for many years, look at the DNA and lo and behold the DNA tells you what you know from history, they’re marrying their relatives. If you don’t have a history just look at the DNA and it’ll tell you what you need to find out.

Jimmy Doherty:: And are there problems caused by inbreeding?

Steve Jones: There certainly can be problems caused by inbreeding. They’re not as severe as that in plants, why that is we don’t know really. But plenty of genetic diseases need two copies of the same damaged gene, and if you’ve only got one copy it’s maybe masked by a normal gene and you’ll be okay. But let’s say you’re the offspring of a cousin marriage, your parents share a grandparent in common, then maybe that grandparent had one copy of the damaged gene therefore you might have two. And there really is an effect of human inbreeding, it leads to an increase in mortality among young children.

Darwin was related to a chap called Francis Galton who really founded in some sense his human genetics. And Darwin’s worries about his marriage were in some ways ahead of their time because they were genetical worries. People now worry about inherited disease, Darwin had that worry and he was particularly worried because he was hoping to marry his cousin, and he had this then vague general feeling that cousin marriage, inbreeding, was a bad thing. Now he had a number of children, many of his children were quite unhealthy and several of them died, and we know that actually inbred marriages, cousin marriages do really cause a certain amount of harm.

In the modern context not very much, but it’s definitely there. The chances of early death for the children of cousins is about twice as much as for unrelated people. And that sounds awful, and certainly for the families involved it is awful, but let’s put it into context. Nowadays in the developed world very, very few babies die young, more babies of cousin marriages die young, but the effect for most people is not important but it’s definitely there. And Darwin, you know, being somebody who liked to find out, he just didn’t think about it and worry about it, he did experiments. He couldn’t do it on people so he did it on plants.