human beings. In a small French scientific journal there appeared in 1946 an astonishing story: A woman came to the attention of a doctor in Nancy when she was having her second child; her first, a daughter, had died in infancy: She 'expressed no surprise on learning that the second child was also a daughter: In her family, she said, no sons were ever born.
Her tale was this: She was the ninth daughter of a sixth daughter: Her mother had no brothers, 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 35
been born in two generations of her family and not one man: 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, the scientists conjectured that the woman and her relatives contained some kind of cytoplasmic gene that feminized every embryo it infected, 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, so that the city 's sex ratio will soon be unbal-
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anced? 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 in 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 and 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. 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 are 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 genes: That all those children are sons and the species will therefore go 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
He saw it as a powerful danger that was
1967.'
liable to drive species extinct suddenly and silently. He wondered what prevented it from happening, if anything did: One solution GENETIC MUTINY AND GENDER
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was 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: It suppresses one only to find it has opened the way to another: For this reason gender determination is a mechanism full of, in the words of Cosmides and Tooby, 'meaningless complexity manifesting unreliability, aberrations, and (from the individual 's point of view) waste.' '
But if the Y chromosome can drive, so can the X. The lemming is a fat arctic mouse famous among cartoonists for apocryphally throwing itself off cliffs in hordes: It is famous among biologists for its tendency to explode in numbers and then collapse again when overcrowding has destroyed its food supply. But it is notable for another reason: It has a peculiar way of determining the gender of its babies: It has three sex chromosomes, W, X, and Y. XY
is a male; XX, WX, and WY are all females. YY cannot survive at all: What has happened is that a mutant form of driving X chromosome, W, has appeared that overrules the masculinizing power of the Y The result is an excess of females. Since this puts males at a premium, you might expect that males would soon evolve the ability to produce more Y-bearing than X-bearing sperm, but they have not done so. Why? At first biologists thought it had something to do with population explosions during which an excess of daughters is a good idea, but recently they have determined that this is unnecessary: The female-biased sex ratio is stable for genetic, not ecological, reasons:'
A male that produces only Y sperm can mate with an XX
female and produce all sons (XY) or with a WX female and produce half sons and half daughters or with a WY female. In the last
case he has only WY daughters because YY sons die: The net result, therefore, is that if he mates with one of each, he will have as many daughters as sons, and all his daughters will be WY
