Cain genes survive in mice and flies by hugging their masks close to them so that they are not likely to be parted by crossing over. But there is one pair of chromosomes that is especially plagued by Cain genes, the ' sex chromosomes,' because these peculiar chromosomes do not engage in crossing over. In people and many other animals, gender is determined by genetic lottery. If you receive a pair of X chromosomes from your parents, you become a female; if you receive an X and a Y, you become a male (unless you are a bird, spider, or butterfly, in which case it is the other way around): Because Y chromosomes contain the genes for determining maleness, they are not compatible with Xs and do not cross over with them. Consequently, a Cain gene on an X chromosome can safely kill the Y chromosome and not risk suicide. It biases the sex ratio of the next generation in favor of females, but that is a cost borne by the whole population equally, whereas the benefit
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of monopolizing the offspring is received by the Cain gene itself—just as in the case of free-riders causing the tragedy of the commons.'
I N PRAISE OF UNILATERAL DISARMAMENT
By and large, however, the common interest of the genes prevails over the ambitions of the outlaws. As Egbert Leigh has put it, 'a parliament of genes' asserts its will. Yet the reader may be getting restless: 'This little tour of the cellular bureaucracy, ' he says, 'fun though it was, has brought us no closer to the question asked at the beginning of the chapter—why there are two genders. '''°
Have patience: The road we have chosen—to seek conflicts between sets of genes—leads to the answer. For gender itself may prove to be a piece of cellular bureaucracy: A male is defined as the gender that produces sperm or pollen: small, mobile, multitudi-nous gametes. A female produces few, large, immobile gametes called eggs. But size is not the only difference between male and female gametes. A much more significant difference is that there are a few genes that come only from the mother: In 1981 two scientists at Harvard whose perspicacity we will reencounter throughout the book, Leda Cosmides and John Tooby, pieced together the history of an even more ambitious genetic rebellion against this parliament of genes, one that forced the evolution of animals and plants into strange new directions and resulted in the invention of two genders.''
So far I have treated all genes as similar in their pattern of inheritance. But this is not quite accurate. When a sperm fertilizes an egg, it donates just one thing to that egg: a bagful of genes called a nucleus. The rest of it stays outside the egg. A few of the father 's genes are left behind because they are not in the nucleus at all; they are in little structures called 'organelles. ' There are two main kinds of organelles, mitochondria, which use oxygen to extract energy from food, and chloroplasts (in plants), which use sunlight to make food from air and water. These organelles are GENETIC MUTINY AND GENDER
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almost certainly the descendants of bacteria that lived inside cells and were 'domesticated ' because their biochemical skills were of use to the host cells. Being descendants of free-living bacteria, they came with their own genes, and they still have many of these genes: Human mitochondria, for example, have thirty-seven genes of their own: To ask, 'Why are there two genders? ' is to ask, 'Why are organelle genes inherited through the maternal line? ' ' Why not just let the sperm 's organelles into the egg, too? Evolution seems to have gone to extraordinary lengths to keep the father 's organelles out: In plants a narrow constriction prevents the father 's organelles from passing into the pollen tube: In animals the sperm is given a sort of strip search as it enters the egg to remove all the organelles: Why should this be?
The answer lies in the exception to this rule: an alga called
As the prince soon discovers, even this severe sentence is insufficient to suppress the quarrel. Had he followed the example of the nuclear genes, he would have killed all the Montagues. The nuclear genes of both father and mother between them arrange that the organelles of the male are slaughtered. It is an advantage (to the male nucleus, not to the male organelles) to be of the type that allows its organelles to be killed, so that a viable offspring results.
So owners of docile, suicidal organelles (in the minus gender) would proliferate. Soon any deviation from a ratio of fifty-fifty killers and victims would benefit the rarer type and cause the ratio to correct itself. Two genders have been invented: killer, which provides the organelles, and victim, which does not.
Laurence Hurst of Oxford uses these arguments to predict that two genders are a consequence of sex by fusion. That is, where sex consists of the fusing of two cells, as in
