by Geoff Swinfield FSG

On 17th January 2013, Steve Jones, the eminent geneticist, gave his presentation Incest and Folk-Dancing: Two things to be avoided at Gresham College, Barnard’s Inn Hall, just off Holborn in London. The Emeritus Professor of Genetics at University College, London, took his very amusing title from the famous quotation by Sir Thomas Beecham. Of course, he mainly concentrated on the former.

He first looked at what makes males so different to females in most species of animals. In most cases, this is due to the inheritance of that very odd structure, the Y-chromosome, which appears to be a former shadow of the X-chromosome from which it has deteriorated over millions of years. For many reasons, especially the testosterone which it generates, it causes male humans to succumb, at a much higher rate than females, to death from virtually every cause of disease and accident. Indeed, men are ten times more likely to be murderers and to be murdered than are women.

Steve then looked at how males and females differ in the relative production of their sex cells. How does their strategy affect the ability to find a mate and successfully reproduce their genetic code for future generations? For some reason, women, and females of all species which partake in sexual reproduction, are prepared to pass only half their genome on to their offspring whilst acting as a conduit for their partner’s share. The benefit is, of course, the inheritance of variation caused by mutation, which ultimately allows the species to evolve through natural selection. Amazingly, Steve quoted, that all the women in the world generate about 400 eggs every second, from which, on average, 5 babies are born. To fertilise those, men are generating “river loads” of sperm, when only a few are needed.

The next part of the presentation looked at how closely we are all related to each other. The chances are that we were something like 6th cousins to the person sitting next to us. Yes we are all descended from those who unknowingly were committing some form of incest! There are just not enough separate ancestors to go around. All our pedigrees show some degree of “inbreeding” through sharing common ancestors on many lines of descent. He illustrated just how far that can go with members of European royal households. In some instances, where over seven generations there should be 128 different ancestors, a number of “royals” have far less than 20, the lowest being just 8!

Why is important to “out-breed”? That will reduce the chance of passing on inherited diseases. In some isolated populations, such as in Finland, where lineages can be traced back to very few ancestors, this has culminated in the appearance in recent generations of “recessive” diseases.

This same lack of a wide choice of mates and inherent “incest” has also produced, in some isolated Italian valleys, the elimination of many of the original surnames which had flourished there. If some men only produce daughters, their family name will be lost. Eventually it is possible for everyone who lives there to have the same surname. In the neighbouring valley, the entire population is called something else!

DNA can now be used to study inbreeding. Using Turi King’s study at Leicester University of the Y-chromosome and surnames, Professor Jones showed that those men who have rarer surnames, such as Attenborough, are very likely to have the same form of the male chromosome. On the other extreme, those with the commonest surnames such as Smith (and Jones) will carry the full range of Ys from all parts of the human species.

We can now study all the other parts of the genome, the autosomes (or non sex-determining chromosomes) through so-called “runs of homozygosity” (ROHs). Looking at the inheritance of identical large blocks of the genetic code within populations, it is possible to measure their level of in or out breeding. Thus amongst the indigenous inhabitants of some islands off the coast of Croatia, there has been a very high level of inbreeding. [Interestingly, that is very close to the area where he went to do his snail experiments in the 1970s. I was able to remind him afterwards that, as an undergraduate, I had been part of the last of those expeditions as long ago as 1972]. Very high levels of ROHs demonstrate that they, as do those from The Orkneys, must have the same common ancestors on many lines of their family tree through inbreeding or varying degrees of hidden incest.

A fascinating insight into this “sin” and its impact on population genetics was thoroughly enjoyed by all those who were lucky enough to be present. We look forward to learning about the similarly wicked effects of folk-dancing on mankind in a future talk.