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In previous posts I discussed some of the peculiarities of X-chromosome gene inheritance and expression and the implications for things like intelligence and Asperger’s syndrome. I pointed out that the evidence is that, thanks to a large number of genes on the X being implicated in intelligence and related cognitive skills, you could say that you inherit your IQ from your mother. This is obviously the case with sons, who receive their one and only X-chromosome from their mothers (with a corresponding Y from their fathers). But it is indirectly true of daughters too, because although they get an X-chromosome from both parents, the X they get from their father is exactly the same one that he got from his mother—his daughter’s paternal grandmother. This makes daughters’ relatedness to their fathers in this respect different to their relatedness to their mothers. The X-chromosome a woman inherits from her mother is a random mix of genes from both of her mother’s Xs, and so does not correspond as a whole with either of them in the way in which her paternal X-chromosome corresponds to her father’s X.
(This also means that the paternal grand-mother is the most closely X-related grand-parent to her son’s daughters. The maternal grandmother’s X-genes are diluted by those of the maternal grandfather in the X the grand-daughters get from their mother, meaning that the X-relatedness of the maternal grandparents to their grand-daughters is only half that of the paternal grandmother. The paternal grandfather, however, is the least related of all the grand-parents to his son’s daughters because he confers no sex chromosome genes on them at all.)
In earlier posts I also explained Lyonization: that is, Mary Lyon’s discovery that normally one X-chromosome is silenced in each cell of a woman’s body so that, just like a man, she expresses only one. This means that, in any particular cell of a woman’s body, either the maternal or the paternal X-chromosome is generally expressed (but we now know that there are some exceptions). However, as I also pointed out in a previous post, we also know that there can be skewed outcomes, with some women expressing much more of one parent’s X-genes than the other’s.
This in turn means that some daughters might be cognitively much more like their fathers than their mothers—especially if the paternal X were disproportionately expressed in their brains. They would be X-chromosome clones of their fathers in this respect because each and every X-gene he had would be inherited and expressed by them. In other words, if sex chromosome expression is skewed in favour of the father’s X in the brain, a woman’s mind could end up surprisingly similar to her father’s in many respects—and, by the same reasoning, surprisingly dissimilar to her mother’s!
Freud emphasized the importance of the relationship between mothers and sons, but in my experience it pales into insignificance in the light of that between many fathers and daughters, who often seem to have a particularly close emotional bond that often intensifies with time. Today we know that identical twins’ psychometrics—and IQs especially—converge as they age, a sure expression of the power of nature over nurture with the passage of time, and perhaps the father-daughter tie is the same. Sigmund Freud’s own relationship with his daughter Anna certainly seems a case in point, as I was able to observe at first hand—at least in the daughter and at the very end of her life. Indeed, perhaps this explains the uncanny experience I had from time to time when, reclining on Anna Freud’s couch and listening to her comments, I felt I was hearing the voice of the prophet speaking from beyond the grave—or was it from her Freudian X-chromosome?
