Why we're having less sex with our (genetic) relatives As a result - and just as in marriages between people with the same family name - the presence of two long and identical copies within any individual is a strong hint of recent shared ancestry. Islanders, from the Orkneys to the Adriatic (together with natives of the Americas, which was, until Columbus, the biggest and most isolated island in the world) have lots of such things - evidence of plenty of sex within the family, no doubt because no other option was available. Across the world, such patterns match those of surnames. A fifth of all Chinese - three hundred million people - share three names, evidence of how connected lineages have become since the titles first appeared millennia ago. In France, the average number of bearers of a particular surname is 17, and in Britain it is 28. In China, the number is 70,000 (which almost reconciles me to being a Jones). As might be expected, certain family labels are associated with particular inherited diseases. But the malign effects of long, doubled-up DNA surnames are, it now appears, much greater, for they reflect the extent to which large sections of the genome as a whole descend from a common ancestor and, as a result, bring many damaged genes together in double copy. They are unduly common in people with colon cancer, autism, Parkinson's disease and other conditions, and doctors have become very interested in scanning patients to test their risk. So astonishing is the advance of technology that this discovery is already becoming out of date. Now it is easy to pick up millions of short doubled-up differences across the whole double helix, rather than in just a few lengthy but untypical segments. A new survey of this kind involved 5,000 random (and supposedly unrelated) Europeans. It revealed hundreds of thousands of previously unknown family links among them, even if one goes back no further than ninth cousins (whose shared ancestor lived at around the time of the French Revolution). There were, for example, around 30,000 predicted fourth-cousin pairs (a shared great-great-great-grandparent). As a result, taking all family ties into account, the person you sat next to on the bus this morning is, on average, likely to be something like your sixth cousin, which means that the two of you probably share at least one ancestor from the time of the Paris Commune. Finns (who have a history separate from that of the rest of the continent) and Ashkenazi Jews are even more likely to have close family ties; while in parts of Pakistan, the average relationship of two random people is that of second cousins, with their common ancestor alive at the time of the fall of France. Sir Thomas Beecham (had he ever met a Pakistani, or even a Welshman) would no doubt have been outraged. But he can begin to cheer up, for in the Western world incest (or at least inbreeding) is on the way out. The proportion of people who identify themselves as of mixed race in Britain has almost doubled in the past couple of decades, and one household in eight contains members of different ethnic origins. For about half of the nation's children with an Afro-Caribbean parent, the other parent is white, so that on these islands the pedigrees of two continents will soon merge. The process actually began long ago: seven Yorkshiremen bear the surname Revis (after Rievaulx Abbey in the county). Each carries a Y chromosome that came from West Africa, perhaps in the 18th century. It is now being joined by millions more. Of course, obstacles to sexual relations among groups have not disappeared. In the United States, black-white unions make up only one in 60 new marriages today, far fewer than in Britain - but even there the incidence has shot up from fewer than one in 1,000 when Barack Obama's parents tied the knot half a century ago. A survey of long-shared blocks of DNA in Americans of different age also shows how the habit of sex with relatives is fading. The oldest of the 800 people tested was 99, the youngest 19. Over the past century or so, the numbers of doubled DNA sections have dropped by about a seventh, and their average length by a quarter. All this reflects changes in patterns of mating - and the increase in sex with strangers - since the invention of the motor car. With the budget airline and the free movement of labour, the decline in relatedness among British partners will become even steeper. One way to test the effect in one's own family is to ask how far apart you and your partner were born, in comparison to your mother and father, your mother and father's parents, and so on. In most cases, the distance has shot up. My wife and I, for example, were born 3,000 miles apart, while my own parents first saw the light in places separated by only 30 miles (I failed to identify the student who once shouted from the back row: "And it shows!"). Time, perhaps, to take up folk dancing.