Twins: Brazil edition

By Razib Khan | March 24, 2011 8:27 pm

A few years ago a story came out about a town populated by Germans in Brazil which exhibited a tendency toward twinning. The combination of Germans, Brazil, and twins, naturally meant that Josef Mengele came into the picture. A more prosaic explanation for the twinning, favored by locals, was that it was something environmental, like their water. The oddity warranted coverage by National Geographic, and you can imagine what the British press did with the story. At first I thought I saw references to elevated frequencies of identical (monozygotic) twins, which would have have been strange indeed. Twinning varies across populations and families, but that variance tends to be of the fraternal (dizygotic) variety. Some of this is heritable, but some of it is clearly due to environment. Specifically, nutritional inputs that increase levels of insulin-like growth factor, which is found in milk and meat (I suspect this explains the higher twinning rate in Northern Europe vis-a-vis Southern Europe). This doesn’t even go into other factors brought on by modernity, such as delayed childbearing and fertility technology.

But in any case, it turned out that the Brazilian twins were not disproportionately identical. Additionally, as expected there was nothing to the Josef Mengele angle. Now The New York Times is reporting that the researcher who debunked some of the wilder claims, as well as the environmental explanation, is going to present her results:

But a group of scientists now says it can rule out such long-rumored possibilities. Ursula Matte, a geneticist in Porto Alegre, Brazil, said a series of DNA tests conducted on about 30 families since 2009 found that a specific gene in the population of Cândido Godói appears more frequently in mothers of twins than in those without. The phenomenon is compounded by a high level of inbreeding among the population, which is composed almost entirely of German-speaking immigrants, she said.

“We analyzed six genes and found one gene that confirms, in this population, a predisposition to the birth of twins,” Dr. Matte said.

We’ll see. I can totally accept that there’s a genetic basis for this disposition, twinning is heritable. But I’m a touch skeptical that there is such a large effect gene (my skepticism would be modulated if I knew what gene the researcher was talking about, so as I could include into the calculus its functional plausibility). We are not a species which gives birth to multiples habitually, and there’s probably a reason for that. It’s clearly biological possible, but it seems likely that in most pre-modern environments it would not have been feasible to raise two children of exactly the same age simultaneously. So a cultural response would have been to kill one of the newborns (I don’t know whether it’s an urban legend or not that in some societies this was practiced because twins were “bad luck”). This “excess” production is wasteful from an evolutionary perspective, so I presume that there was some lock-down on “hyper-fertility” genes. Singletons has long been our lineage’s ancestral state (only one family of primates produces twins commonly), so a gene of large effect which reorders probabilities of multiples so much seems unlikely to me. I would actually increase the probability that this gene was the real culprit if there was a lot of evidence of negative side effects (e.g., high miscarriage rates), which one might expect when you change the function of an important genetic switch. But who knows what could be happening in highly inbred families, whose genetic backgrounds are special to themselves.

CATEGORIZED UNDER: Genetics, Genomics
  • gcochran

    There are several such genes of large effect in sheep.

  • http://nomadicjoe.blogspot.com/ NomadView

    Here is an interesting story about a Russian example of twinning too.
    http://goo.gl/fXJ0o

  • Illinoisjoe

    I have also heard about infanticide of twins.. I recalled something from “Things Fall Apart” by Chinua Achebe where twins were killed, though of course this is a work of fiction. However, a search for the word “twins” on wikipedia’s infanticide page would suggest this is more than an urban legend:
    “For the Maidu native Americans twins were so dangerous that they not only killed them, but the mother as well.”
    and this:
    “In Africa some children were killed because of fear that they were an evil omen or because they were considered unlucky. Twins were usually put to death in Arebo; as well as by the Nama Hottentots of South West Africa; in the Lake Victoria Nyanza region; by the Tswana in Portuguese East Africa; among the Ilso and Igbo people of Nigeria; and by the !Kung Bushmen of the Kalahari Desert. The Kikuyu, Kenya’s most populous ethnic group, practiced ritual killing of twins.”

  • jb

    According to “The Old Way”, by Elizabeth Marshall Thomas, the Kalahari Bushmen did not kill both twins. Rather they would abandon one of them, and would feel very sad that they had to do this.

  • Douglas Knight

    There are lots of populations with high twinning rates. It seems suspicious to me that a gene for twinning should be first discovered in this photogenic population.

  • Pingback: Linkage is Good for You: Regrets Edition()

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This blog is about evolution, genetics, genomics and their interstices. Please beware that comments are aggressively moderated. Uncivil or churlish comments will likely get you banned immediately, so make any contribution count!

About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com

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