It used to be so simple: because identical twins share the same genes, any significant differences between them—for example in IQ or behaviour—had to be non-genetic, and therefore environmental. Indeed, the nature/nurture controversy took it as axiomatic that this was so, and the concept of heritability was defined in these terms. But now we know better: identical twins can differ genetically—at least if they are female.
It goes back to my previous post about sex differences. Because female mammals have two X chromosomes and males only one, one X is largely inactivated in each female cell to avoid the double-dosing with X-chromosome gene products that would otherwise result. This makes a woman a genetic chimera where X-chromosome gene-expression is concerned because half her cells normally express her father’s X and half her mother’s.
In 1875, Darwin described a disorder that appeared in each generation of one family’s male members, affecting some but sparing others: “...small and weak incisor teeth...very little hair on the body...excessive dryness of the skin...Though the daughters in the … family were never affected, they transmit the tendency to their sons; and no case has occurred of a son transmitting it to his sons.” Today we know this to be Anhidrotic Ectodermal Dysplasia (AED), a disorder involving sweat glands among other things which affects males and females differently. Because it is an X-linked disorder, males who suffer from it have no sweat glands at all because they express their one and only X in all their cells. Affected females with the AED gene on only one X are however chimeric for symptoms. For example, it is perfectly possible for such a case to have one armpit that sweats and one that doesn’t!
As I also pointed out in the previous post, there are at least 150 genes linked to intelligence on the X chromosome, and verbal IQ is definitely known to be X-linked. A recent study found that compared to male, female identical twins vary more on measures of social behaviour and verbal ability thanks to differential X-inactivation. This has a number of important implications. One is that as medical geneticists have long suspected, you get your intelligence predominantly from your mother, not your father—especially if you are male. It has been pointed out, for example, that Charles Darwin inherited X chromosome genes from his maternal grandfather, Josiah Wedgewood, which probably explain his gifts better than the Y chromosome he got from his paternal grandfather, Erasmus Darwin. (However, as the diagram suggests, Erasmus Darwin may indeed have been the origin of the remarkable intelligence of Darwin’s cousin, Francis Galton.)
Another important implication is that differences between identical twins are not necessarily only attributable to nurture or environmental factors, but may be genetic. As a consequence of this, the heritability of intelligence has probably been systematically under-estimated. Indeed, the fact that males have only one X-chromosome to rely on—the one they got from their mother—almost certainly explains not simply why so many more males than females have IQs below 70 but also why 20% more males than females have IQs above 140. Greater variability is inevitable if only one X chromosome is being expressed as compared with the two on which a woman can normally rely. Furthermore, this in itself could explain why some men outperform women intellectually in some respects: ironically perhaps, these high performing males are expressing a single outstandingly gifted X-chromosome that they got from their mothers.