The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [54]
Her tale was this. She was the ninth daughter of a sixth daughter. Her mother had no brothers and nor did she. Her eight sisters had thirty-seven daughters and no sons. Her five aunts had eighteen daughters and no sons. In all, seventy-two women had been born in two generations of her family and not one man.34
That such a thing should happen by chance is possible, but amazingly unlikely: less than one chance in a thousand billion billion. The two French scientists who described the case, R. Lienhart and H. Vermelin, also ruled out selective spontaneous abortion of males on the grounds that there were no signs of it. Indeed, many of the women were unusually fecund. One had twelve daughters, two had nine and one had eight. Instead they conjectured that the woman and her relatives contained some kind of cytoplasmic gene that feminized every embryo it found itself within, regardless of the sex chromosomes present. (There is no evidence, incidentally, that virgin birth was involved. The woman’s eldest sister was a celibate nun and childless.)
The case of Madame B, as she was described, is tantalizing in the extreme. Did her daughters and nieces have only daughters? Did her first cousins? Is there still, in Nancy, an ever-growing dynasty of women, such that the city’s sex ratio will soon be unbalanced? Was the explanation proffered by the French doctors the right one? If so, what was the gene and wherein did it live? It might have been in a parasite or an organelle. How did it work? We may never know.
The Alphabetical Battle of the Lemmings
With the exception of some female inhabitants of the city of Nancy, the gender of a human being is determined by his or her sex chromosomes. When you were conceived, your mother’s egg was chased by two kinds of your father’s sperm, one containing an X chromosome, one containing a Y chromosome. Whichever got there first decided your gender. Among mammals, birds, most other animals and many plants, this is the usual way of going about things: gender is determined genetically, by sex chromosomes. Those with an X and a Y are male, those with two Xs are female.
But even the invention of sex chromosomes, and their success in largely suppressing the rebellion of cytoplasmic genes, did not succeed in making life harmonious in the society of genes. For the sex chromosomes themselves began to have an interest in the gender of their owners’ children. In man, for instance, the genes that control gender are on the Y chromosome. Half of a man’s sperm are X-carriers, and half Y-carriers. To father a daughter, the man must fertilize his mate with an X-carrier. In doing so he passes none of the Y’s genes to her. From the Y’s point of view, his daughter is unrelated to him. Therefore, a Y gene that causes the death of all the man’s X-bearing sperm and ensures its own monopoly of the man’s children will thrive at the expense of all other kinds of Y gene. That all those children are sons and the species will therefore became extinct matters not in the least to the Y: he has no foresight.
This phenomenon of the ‘driving Y’ was first predicted by Bill Hamilton in 1967,35 who saw it as a powerful danger, that was liable to drive species extinct suddenly and silently. He wondered what prevented it happening, if anything did. One solution is to gag the Y chromosome, removing all but its gender-determining role. Indeed, Y chromosomes are kept in a kind of house arrest most of the time: only a few of their genes are expressed and the rest are entirely silent. In many species gender is determined not by the Y chromosome but by the ratio of the number of X chromosomes to the number of ordinary chromosomes. One X fails to masculinize a bird, two succeed: and in most birds, the Y chromosome has withered away altogether.
The Red Queen is at work. Far from settling down to a fair and reasonable way of determining gender, nature has to face an infinite series of rebellions.