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.
These laws of genetic transmission have major implications for family lineages. When it comes to grandparents, women are always most closely X-related to their paternal grandmother and less related to their paternal grandfather. Consider how this plays out in the current British royal family: The late Diana, Princess of Wales, will be more closely related to any daughter born to William and Kate than will Kate's parents, thanks to William's passing on his single X from her. Kate's mother's X genes passed on to a granddaughter, by contrast, will be diluted by those of Kate's father in the X this girl would receive from Kate, meaning that the X-relatedness of the Middletons to any granddaughter would be half that of Princess Diana. Prince Charles, however, would be the least related of all four grandparents to Prince William's daughters because he confers no sex chromosome genes on them.
Of course, if William and Kate produce a son, the situation is reversed, and now Prince Charles is most closely related to his grandson, via his Y chromosome. Princess Diana will have no sex-chromosome relatedness to William and Kate's sons because the X she bequeathed William will not be passed on to the grandsons.
Calculations of X-relatedness may seem abstract, but they have probably played a huge role in European history, thanks to the fact that Queen Victoria passed on hemophilia, an X-chromosome disorder that was in the past fatal to males. Victoria and her female descendants were protected by a second, unaffected X, but princes in several European royal houses—not least the Romanovs—were affected, with disastrous consequences for successions based on male primogeniture.
The X in Sex
Not only are X chromosomes bequeathed and inherited differently, depending on whether you are male or female, they also have different patterns of expression in the body. For example, 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, that affects males and females differently. Because AED is carried on an X chromosome, affected males have no sweat glands whatsoever. They express their one and only X in all their cells. Affected females with the AED gene on only one X have different patterns of expression because areas of their body randomly express one or the other of their two X chromosomes. It is perfectly possible for an affected woman to have one armpit that sweats and one that doesn't.
