Steve Jones - The Language of the Genes

Conflict between males for the attention of females is obvious, but there are also plenty of chances for disagreement between the sexes. In some animals, the reluctance of females to accept a new mate, persistent though he might be, arises because males invest less in bringing up offspring. It pays them to mate and run; to try and father as many children with as many females as possible. Females need to be more cautious. As it costs so much to produce a child they should choose the male who will be the best father and reject the rest.

The divergence of interest is sometimes obvious. Some males kill a mother's brood by another male with the aim of making her available to themselves. Among the langur monkeys, most of the young die for this reason. Some species even have a form of prenatal cannibalism. Pregnant female horses exposed to a new male reabsorb their foetuses, a behaviour which may have evolved because of the near certainty that if born they will be killed.

Humans reveal the intersexual struggle in less blatant ways. Their battle is an economic rather than a mortal one. If tribal peoples are any guide, societies with private property are more polygamous, as women prefer the better-endowed as mates. When wealth is concentrated into few hands, society becomes more like that of a gorilla, with the richest males monopolising the females. The philoprogenitive (and opulent) Moulay Ismail the Bloodthirsty of Morocco admitted to 888 children. We in the West now seem to be moving towards the chimpanzees, as most men have at least a fair chance of a Ms Right, but in most societies success is still related to wealth. Among the Kip-sigis people of south-west Kenya a wealthy man may have as many as a dozen wives and eighty children. The more land a man has the more wives he obtains and the poorest males leave the community as teenagers and have no children at all. All women, in contrast, tend to have families of about the same size. In Britain, too, men from higher social groups have more partners than do those less well off. An economic conflict between the sexes means that men provide the capital and women choose where to invest.

The battle of the sexes may explain another unusual attribute of human reproduction. Women are the only female primates who do not make it obvious when they are most fertile. Most female primates advertise the two or three days in each cycle when they are able to conceive*. Often, this is accompanied by a frenzy of copulation with a series of males. Before modern medicine, most women (and all men) were unaware of when the fertile period was. Women's reproductive coyness might reflect the change in the economic relation of the sexes which came with the origin of society. It could, some suggest, be an attempt to resolve the conflict between male promiscuity and the female's need to ensure the care of her children. By concealing when she is fertile she ensures constant attention from her mate. If he is not sure when she can conceive then he dare not leave her for a new woman in case another male takes advantage of his absence. This is historical speculation with no evidence for or against it — and, as so often with theories of history, several other interpretations are possible.

Males do, needless to say, contribute to the care of their children, but in most societies the sexes differ in their commitment to the next generation. The mother is usually left holding the baby when a relationship breaks up. The difference can be subtle. Many genetic tests can tell parents whether they carry a harmful gene and whether it is wise for them to plan to have children. In a few cases, the test also tells the parents themselves that they are at risk of illness themselves. Huntington's disease is of this kind. Twice as many women as men volunteer for a test, perhaps because their concern for their potential child's future is greater than that for their own peace of mind.

The battle of the sexes is often seen as regrettable but unavoidable, but most people assume that the bonds between mother and child.nv driven by mutual devotion. To tin- cold eye ot the biologist rhc transaction between generations is also filled with conflict, with many chances for i he two parties to exploit each other. It is in the child's interest to gain as much attention as possible from its mother. The mother's concern is to provide as little as will allow her progeny to survive. If she is too generous to one, the next may suffer.

Such confrontations, deplorable as they seem, are the commonplace of the animal world. There has grown up in biology the comforting supposition that nature is not really red in tooth and claw and that animals rarely do much harm to other members of their own species. The battle for reproductive success shows how wrong this is. Eagles lay several eggs. If food is plentiful all the chicks are fed, but any shortage means that the last to hatch is allowed to starve or is killed by its sibs. Rats, mice and other mammals often eat all their young when food gets short (a habit known as kronism, after the Greek deity Kronos, who devoured his own children).

Any mother is certain that all her children (first, second or third born) carry her own genes, but it is quite possible (and in many animals almost guaranteed) that the father of her first child will not be the same as that of her later offspring. As Aristotle put it: 'This is the reason why mothers are more devoted to their children than fathers: it is that they suffer more in giving them birth and are more certain that they are their own'. The conflicts of interest involved, and the differences in the investment of each sex in their young, may help to explain certain strange patterns of inheritance.

Much to the surprise of geneticists the effects of a particular gene sometimes depends on whether it is passed on by mother or by father. This effect, 'genomic imprinting' as it is known,is quite different from sex linkage, as thegenes involved may be on any chromosome. Each sex seems to stamp its personality on what it transmits. The DNA itself is not permanently altered, but its effects on those who inherit it depend on which parent it came from. A gene passed on by a father to his daughter differs in its impact from that of the same gene passed by her to her own children. The DNA is 'marked' as it is conveyed through sperm or egg and the mark reversed when tin- line of transmission changes from one sex to another.

Each embryo contains both maternal and paternal DNA. If we use the (rather dubious) metaphor that every gene acts in its own interests, it pays those that come from the father to extract as much as possible from the mother in which they find themselves, irrespective of any damage which this does to her or to subsequent children, as later offspring may well carry genes from a different father. The first father loses nothing by exploiting his mate as much as he can as she will bear no more of his children. The mother, in contrast, needs to ensure that further attempts to pass on her own heritage are not jeopardised by the avarice of her firstborn. The difference in behaviour of thesame gene when transmitted through fathers or through mothers hence arises from paternal greed.

In mice, the genes that make the membranes through which the foetus feeds are more active if they come from the father; and the effect is so strong that the placenta itself has been described as a parasite forced on the mother by the father. Those passed on by the male parentalso tend to increase the size of the tongue (which is, of course, used in suckling). Genes for human disease show the same effect. Some foetuses by accident inherit two copies of a gene that promotes growth. They become abnormally large only if both copies come from the father. In normal foetuses just the paternal copy is switched on as further evidence of the father's interest in his child extracting the most it can from its mother. Two rare genetic diseases (the Prader-Willi and Angelman syndromes) were once thought to he different as their symptoms are distinct. In fact they arc duo to the same mutation, a deletion of a short segment of chromosome fifteen which damages the gene involved in imprinting paternal identity. The differences depend on whether it is passed on by father or by mother. Children with Prader-Willi syndrome (whose abnormal chromosome comes from their father) suckle hard, become obsessively interested in food and are fat, while Angelman children (who receive the same structure from their mother) are thin or of normal weight, but have quite severe nervous symptoms, such as epilepsy, a tendency towards constant laughter, and a fascination with water. In the latter disease, the paternal copy of the gene is silenced in parts of the brain, perhaps explaining the mental illness. Enthusiasts for conflict suggest that even a baby's cries are an attempt to manipulate its mother to provide more food and that the mother retaliates by secreting in her milk substances similar to those used as sedatives by doctors. Whether or not this is true, it is clear that once sex has evolved it has some unexpected effects on the lives of the creatures who practise it. Without sex there would be almost no evolution and no genetics. Our universal fascination with the subject may, one day, solve the most important sexual problem of all — why we bother in the first place.