Saturday 6 April 2013

Fancy some sex, cousin?

Is mixing races an intrinsically good thing, because it reduces the number of relatives held in common? Professor Steve Jones, probably Britain’s best known geneticist, apparently argues so, in an elegant and witty Telegraph column:

Here are some illustrative paragraphs:

the malign effects of long, doubled-up DNA ……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.

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.”

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.

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. 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.

Comment: It would seem to be clear from this article that a willingness to marry across “continents of origin” will reduce some of the unpleasant effects of inbreeding. Put like that, some readers might consider that such relationships are a public duty, diluting the deleterious effects of quasi-incestuous parochial pairings. Even driving for 100 miles to find a partner seems to convey a benefit.

Does this present a balanced picture? Well, it seems to suggest that relatedness carries a clear disadvantage, with no countervailing advantages. Ashkenazi Jews and Finns are lumped together with Pakistanis, without reference to intellectual and scholastic abilities, or any behavioural differences.  Let us consider the matter by way of some simple comparisons.

The scientific and cultural achievements of Ashkenazi Jews are legendary, and very well documented (for example, Richard Lynn “The Chosen People: A study of Jewish Intelligence and Achievement, Washington Summit Press, 2011). Sephardic Jews are a step behind, but well above other contenders.  Jews have won 139 Nobel Prizes in science (Chemistry 33, Medicine 56, Physics 50). The latter subject, in particular, would have been left in a very different state without Albert Einstein, Neils Bohr, Wolfgang Pauli, Richard Feynman, and Hans Bethe.  (I have left out mention of my old friend Joseph Rotblat, because despite being a physicist good enough to work on the Manhattan Project, his Nobel Prize was for peace. Also, it allows me to re-tell the Israeli joke about the 1978 Peace prize: “Have you heard that Menachem Begin has won the Nobel Prize for Physics?” “For Physics? I thought it was for Peace!”  “No, no, no. His qualifications in Physics were better”). Anyway, assuming an Ashkenazi Jewish population of 10 million, that gives them 139 Nobels per 10 million or 107 per 10 million for all Jews. At a mundane and worldly level, they tend to be prosperous.

Finns come third in the world in the Programme for International Student Assessment 2009 rankings (behind Shanghai and Korea).  They have got two Science Nobel prizes which converts to 3.8 per 10 million. They tend to be prosperous.

Pakistanis do not participate in PISA. One Pakistani has won a Nobel in Physics, the only Muslim to have done so.  Pakistan gets a per capita score for science Nobels of 0.05 per 10 million. They tend to be poor.

I think you will agree that these three populations differ considerably in their scholastic achievements. They also differ in population size.

Ashkenazi Jews            10 million*    
Finns                             5.5 million;
Pakistanis                    182 million
*There are roughly 13 million Jews world-wide, but far fewer in each nation state: Israel 6, United States 5, Europe2 and Canada .4

This gives us a clue as to what is going on as regards inbreeding. Whereas the first two could be seen as being restricted by population size, that is not the case for the 182 million Pakistanis, who should have no shortage of potential partners. They should be free of inbreeding, so long as they can walk several miles to the next village. The problem is that Pakistanis practice first cousin marriage. This is not a good idea. Europeans have usually avoided it. The link below shows the global distribution of consanguinity.

The Pakistanis should be able to free themselves from the risk of genetic disorders by avoiding their first cousins. The Ashkenazis, on the other hand, who have already avoided their first cousins, but mostly chose partners from among other Jews, have decided to participate in genetic studies aimed at tracing the genes which cause unpleasant neurological disorders, and taking steps to eliminate them, probably by screening foetuses. They hope to avoid eliminating genes which lead to high intelligence, though those cannot be identified at the moment (but are being searched for by Prof Robert Plomin).

A more balance presentation would be to say that some relatedness is not bad of itself. The genetic code carries good and bad messages. Restricted populations have an increased rate of genetic disorders. Cousin marriages almost guarantee an even higher rate of genetic disorders, and those are avoidable.  It is misleading to consider the genetic risks of relatedness without considering how it comes about, and without looking at the benefits of positive characteristics in relatives.

Avoid first cousins, but otherwise marry whom you choose, even among your own genetic group.


  1. I take it that even first-cousin marriages aren't too bad, if indulged in only occasionally - it's doing it generation after generation that's the trouble, is it?

  2. Yes, the repetitions increase the risk.