Psychology Today x chromosome

In the early 1980s I met and began an unofficial training with Anna Freud—Sigmund Freud's youngest daughter, and his only child to follow him into psychoanalysis. I was a young social scientist who had been carrying out a self-analysis for some years.

Anna Freud's couch was a daybed on which I lay, with her seated in a chair at its head. On one or two occasions I couldn't help but think that the voice I heard coming from her chair was in fact that of her father, speaking to me from beyond the grave.

I can even recall her exact words in one case. I had been free-associating about my attempt to analyze myself when Anna Freud remarked, "In your self-analysis you sank a deep but narrow shaft into your unconscious. Here we clear the whole area, layer by layer." This produced a spine-tingling reaction in me, and I surprised Miss Freud (as I called her) by stating that her remark reminded me of her father, because he was particularly fond of archaeological metaphors in his published writings. Most people would simply attribute her statement to the influence of her father's writing on her own choice of words. Thirty years ago, I would probably have said the same. But today, having spent decades researching the links between genetics and psychology, I can offer a different hypothesis, one that goes to the core of all we now know about the inheritance and expression of genes in the brain.

The Royal X

Everyone inherits 23 chromosomes from each parent, 46 in all, making 23 matched pairs—with one exception. One pair comprises the chromosomes that determine sex. Female mammals get an X chromosome from each parent, but males receive an X from their mother and a Y sex chromosome from their father.

The X chromosome a woman inherits from her mother is, like any other chromosome, a random mix of genes from both of her mother's Xs, and so does not correspond as a whole with either of her mother's X chromosomes. By contrast, the X a woman inherits from her father is his one and only X chromosome, complete and undiluted. This means that a father is twice as closely related to his daughter via his X chromosome genes as is her mother. To put it another way: Any X gene in a mother has a 50/50 chance of being inherited by her daughter, but every X gene in a father is certain to be passed on to a daughter.