Identical twins not… err… identical?
Posted by gregdowney on April 4, 2008
Although we’ve been accused of hatin’ on twin studies, I admit that I find twins pretty fascinating, mostly because they attract all kinds of magical thinking, not just within traditional Yoruba cosmology, but also in the West. My favorite pair of twins has to be the ‘Jim twins,’ a pair of identical twins discussed in an article in Time Magazine.
Like many identical twins reared apart, Jim Lewis and Jim Springer found they had been leading eerily similar lives. Separated four weeks after birth in 1940, the Jim twins grew up 45 miles apart in Ohio and were reunited in 1979. Eventually they discovered that both drove the same model blue Chevrolet, chain-smoked Salems, chewed their fingernails and owned dogs named Toy. Each had spent a good deal of time vacationing at the same three-block strip of beach in Florida. More important, when tested for such personality traits as flexibility, self-control and sociability, the twins responded almost exactly alike.
The Jim twins are great; they have given me hours of fun just going over the possible genetic roots for their similarity: ‘Oh my god, we’ve got a shared gene for naming our dogs “Toy” and another one for marrying women named “Linda” and, when that didn’t work out, marrying a second wife called “Betty.”‘ (A similarity that the Time Magazine article doesn’t explore.) I mean, I just can’t stop laughing when I think about how lovers of this sort of data actually think that some sort of gene directs people to go vacation in a particular spot or naming a son either James Alan or James Allan (Ooooh, so close… it must be shared genes!). But I digress… I just love the Jim twins’ story so much (see also Jonathan Marks’ book, What It Means to Be 98% Chimpanzee, where he writes that, ‘In the world of twin studies the unscrupulous and the credulous symbiotically plumb the depths of contemporary pseudoscience.’ He’s way funnier than I am writing about this stuff in pieces like ‘Folk Heredity,’ especially the section on ‘hereditarianism’.)
But now we’ve got an interesting piece from Scientific American, Identical Twins’ Genes Are Not Identical, which explores the possibility that identical twins aren’t perfect genetic copies of each other. As the article reports:
Geneticist Carl Bruder of the University of Alabama at Birmingham, and his colleagues closely compared the genomes of 19 sets of adult identical twins. In some cases, one twin’s DNA differed from the other’s at various points on their genomes. At these sites of genetic divergence, one bore a different number of copies of the same gene, a genetic state called copy number variants.
Although I may have fun with twin studies sometimes, in fact, this is a really interesting finding for a lot of reasons, some of which have nothing to do with twin studies. And by that, it’s not just the possibility that thriller plots which suggest one twin being framed for a crime by his unknown, lost identical evil twin will have to be reconsidered.
For one thing, if two identical twins can have different genomes, then the genome is not 100% set and unchangeable at conception. The researchers highlight this:
Bruder speculates that such variation is a natural occurrence that accumulates with age in everyone. “I believe that the genome that you’re born with is not the genome that you die with—at least not for all the cells in your body,” he says…. The differences between identical twins increase as they age, because environmentally triggered changes accumulate. But twins can also begin their lives with differences, according to Bruder’s study, and that calls into question their very name.
This would seem to be part of the creation of what I heard called ‘post-neo-Darwinist’ evolutionary theory at the last American Anthropology Association meetings. That is, ingrained in some neo-Darwinist models is the assumption that genetic inheritance is absolutely fixed at conception and does not change until death. This is one of the things that makes genetic change, even the emergence of new species, difficult to explain. With only mutation at conception or original transcription (in eggs or sperm, in sexual species) to look to when we explain variation, the mechanisms seem very slight to create all the variation that we need.
Now, there are a number of new mechanisms emerging to suggest ways that human genomes may not be entirely fixed. For example, there’s some fascinating research that I was reading about recently that suggests that most organisms’ genomes are loaded with endogenous retrovirus DNA (see Persisting Viruses Could Play Role in Driving Host Evolution, by Luis P. Villarreal). In fact, some research suggests that placental species owe their existence to enogenous retroviruses in their DNA and that they must be activated so that they can suppress the mother’s immune response to the embryo.
But non-identical twins are not directly linked to endogenous retroviruses (that’s not what I’m saying). I’m only pointing out that neo-Darwinist orthodoxy, at least a fervent belief that genes are unalterable, is finding that it needs to be modified in intriguing ways due to a host of diverse research findings. These studies of non-identical twin DNA in ‘identical’ twins merely confirms this necessity.