The Language of the Genes

by Steve Jones

  Smokers can choose whether to poison themselves but others are not so lucky and fall, through no fault of their own, into the pit dug by their genes. Liver cancer is the fifth commonest cancer in the world with its capital in Africa and in China. The immediate cause is aflatoxin; a chemical made by the moulds that grow on badly stored foods such as peanuts, rice, beans and other staples of the tropical diet. They destroy the immune system, stunt growth, and cause cancer. Those with the disease have a new mutation in a gene whose normal role is to prevent cells from uncontrolled division. The mutation is of the kind produced by aflatoxin in the laboratory and the peoples of these regions have high levels of the poison in their blood. Improvements in food storage could control liver cancer. Poverty means that even this may not be achieved.

  Moves are afoot to protect at least some of those at risk. Heavy smokers — about one in ten of whom will develop lung cancer — are given vitamin A in the hope of reducing the effects of mutations in lung cells. Those who inherit a gene that predisposes to colon cancer are treated with aspirin before they develop symptoms as this might reduce its effects. Such illnesses are sometimes seen as a kind of programmed Nemesis about which nothing can be done. An appreciation of the role of the environment shows this to be untrue.

  Disorders such as cancer and heart disease do run in families but their inheritance is hard to study. Many genes are involved and the circumstances hui-il by those at risk play a part. One way of exploring ilu-m is to use twins, nature's own experiment in human *****.

  Twins are of two kinds, identical and unit identical. Non-identical twins come from the fertilisation of two eggs by two sperm (and now and again turn out to have different fathers). Such twins have half their genes in common and are no more similar than are brothers or sisters. Their situation is — yet again — described in that fount of early genetics, the Old Testament. Jacob and Esau were twins; but 'Esau was a cunning hunter, a man of the fields; and Jacob was a plain man, dwelling in tents'. They looked quite different- 'Behold, Esau my brother is an hairy man, and I am a smooth man' — and even had different manners of speech: 'The voice is Jacob's voice, but the hands are the hands of Esau.'

  Such twins are not uncommon. In marmoset monkeys most births are of this kind. For no obvious reason, their numbers vary from place to place. In Europe, about eight births per thousand are of fraternal twins. France has rather fewer and Spain rather more than the average. Among the Yoruba, in Nigeria, the figure is five times higher. Older mothers tend to have more twins, as do those who have already had several children.

  Identical twins are rarer, at about four per thousand births, a rate which does not change much from place to place. In few mammals are they common, but the armadillo always gives birth to identical quadruplets. Identical twins result from the division of an egg which has already been fertilised. They share all their genes and have long been a source of legend. Castor and Pollux, the heavenly twins, were identical as were their equivalents in Germanic legend, liuldur and I loilur (not to speak of Romulus and Krmus, the founders of Rome).

  Twins can be used in several ways to study nature and nurture. The simplest (but by far the least common) is to find identical twins separated at birth and brought up in different households. If a character is under genetic control the twins should be the same in spite of their contrary circumstances. If environment is more important, each twin should grow to resemble the family with which they spent their childhood.

  This simple plot is the basis of a great deal of fiction, in science as much as in literature. Many studies have claimed to show that identical twins reared apart were similar in size, weight or sexual orientation, but much of this work was unreliable. Often, the adoptive families were similar in social position, or the twins knew each other as they grew up. Twins who believed themselves to be identical turned out to be fraternal when blood tests were used. Even worse, there have been persistent accusations of fraud in such work. All this means that most of the older research on twins reared apart has been discarded. Even so, new work does show that some traits of personality — aggression, introversion and so on — have a genetic component. This does not, of course, mean that nurture can be disregarded. An intrinsically violent man may be calm until he is given a chance to prove his genotype by joining the army.

  A more subtle approach involves a comparison of the degree of similarity of identical twins with that of frater-nals. As both kinds of twin are brought up within the same household the extent to which they share an environment is, or so it appears, the same. Any greater resemblance of identical twins to each other must then, it seems, show genes are involved.

  This approach could be powerful but has its own problems. Both types of twin are brought up together, but identicals may copy each other's behaviour. That makes them appear similar for reasons unconnected with biology. The fact of being identical twins — perhaps with similar names and dressed in identical clothes — may predispose to mental disease.

  One of the fundamental assumptions ol twin studies — that identicals and non-identicals differ only in ilu- extent to which they share genes — is not always justified. Lite before birth can be tough; and more so for identic;)) twins than anyone else. Many illnesses of adult onset — heart disease being one — are, at least in part, due to difficult conditions in the course of the pregnancy. Twin pregnancies are always more of a strain than those of a single foetus. As a result, a shared and hostile environment may impose more similarity on a pair of twins than expected on genetic grounds.

  Identical twins come in two forms. AH arise from the splitting of an early embryo. Some are mirror-images of their sib; to look at one is to see the other in a mirror. Such individuals divided quite late in development, when the left-right pattern of the embryo had been set down. A late split increases the chance that the twins will share a common placenta and will have to fight for a share of their mother's blood. Such twins survive much less well than those who have a placenta each and the survivors are born ten days or so earlier than those with a less difficult time before they are born. So intense is their struggle that one may steal blood from the other, so that one grows up large while the other is small and anaemic. To complicate matters further, some non-identical twins — no more similar in their genes than are brothers or sisters — also exchange cells early in development, so that each is a chi-maera, made up in part of their sibling's tissue.

  Nevertheless, to compare the two kinds of twin has had its successes. Members of a pair of identical twins are twice as likely to suffer from coronary heart disease than are those of a pair of fraternals. For diabetes, the figure is five times. Even tuberculosis is shared to a greater extent between identical than fraternal twins as a hint of an inherited susceptibility. Other characters, such the age when a baby first sits up, are also more similar for identical twins.

  The argument about nature and nurture is of more than scientific interest. It has been rehearsed endlessly by those with one or other political axe to grind. Genetics once used an axe sharpened in the fires of Social Darwinism. Now that it has hit the headlines, there is a new acceptance of biological theories of human behaviour. Arson, traditionalism and even zest for life have all been blamed on the DNA. The nineteen-sixties were the decade of caring and a child's inability to concentrate was blamed on poor teachers. Then there was the 'working-mother syndrome' in which a parent's absence was held to be at fault. Now some psychologists have invented a whole series of behavioural ills coded in the genes while others place renewed weight on friends (and not parents) as the main agent of a child's development.

  Psychology's obsessive need to dissect biology from experience is alive, well and as simplistic as it ever was. One study finds that students with hay fever are unusually shy. This proves that 'there is a small group of people who inherit a set of genes that predispose them to hay fever and shyness'. That is naive; but family and adoption studies do suggest that some aspects of personality, from introversion to the speed of response to a s
ound has an inherited component. From there to the discovery of any genes involved is a long step; but psychologists have not been shy about taking it. Announcements of the discovery of single genes for manic depression, schizophrenia and alcoholism have all quietly been withdrawn.

  One form of behaviour has always raised passions about gene and environment but makes an excellent case that genetics and social attitudes ****- little to do with each other. Homosexual attraction is almost universal at some time in every lifetime. Some people continue in prefer their own sex. Exclusive homosexuality is a convenient subject of study for those interested in the genetics of human conduct as it is easy to identify, quite common, and no longer much concealed.

  One study of American male homosexuals hinted at an association between such behaviour and a group of genes near the tip of the X chromosome. First, the brothers of gay men were more likely to be gay than are males in the general population. This does not in itself say much, as brothers share an environment as well as genes. However, gay men's relatives on the mother's side were more liable to be gay than were those on the father's, suggesting that the trait is passed through females. Again, this is not proof of an innate predisposition (even if it implies a possible gene on the X chromosome). The best evidence seemed to come from the X chromosomes of pairs of homosexual brothers. Most who took part in the study shared a particular segment of DNA towards one end of that chromosome. Somewhere in its hundred or more genes may, it was suggested, be one that inclines some carriers to that form of sexual behaviour.

  After an initial burst of publicity, the result proved hard to replicate (as is often the case for such characters, in which different genes might be involved in different families) and the simple idea of a 'gay gene' is now dismissed. Whatever the science, the main interest lay in the response by some — but not all — of the gay community. Many, it transpired, were happy to use biology as a justification for their way of life. The idea that sexual preference was inherited meant, they concluded, that sexuality was not contagious and that battles by bigots to dismiss homosexual teachers were not justified. More important, it gave a welcome sense of separation: of a shared difference that was present for reasons beyond individual control. All this disconcerted the many biologists who had spent years fighting the idea that sexual preference, crime or poverty are inborn and cannot be altered by social means.

  This new hereditarian orthodoxy, like the old liberalism, asks too much of biology. It echoes a forgotten dispute of the 1930s. The German geneticist Theobald Lang claimed to have found that the sisters of gay men had somewhat masculine characters, and that male homosexuality might therefore be inherited. Whatever the accuracy of his claim that hint of a 'gay gene' gave rise to two quite opposed (albeit equally logical) responses. The Nazis, needless to say, took the brutal view: 'they are not poor sick people to be treated; they are enemies of the state to be eliminated!' In contrast — and faced with the same information — the socialist medical association (then in exile) wrote that 'Homosexuality is inborn and not subject to the free will of the individuals who come into the world with this inversion. The laws against it should be abolished.'

  Like some members of today's gay community, both left and right felt that if that behaviour was innate it must be outside the control of those who display it. Each political group saw its response — eugenic sterilisation versus liberal legalisation — as consistent. Neither asked what is meant by a gene 'for' something, homosexuality included. The story of the German 'gay gene' points up the irrelevance of genetics to political opinion. Whatever inherited basis a character may have, preconceived views about its merits will not be changed by science.

  Nowhere is the difficulty of separating science from politics, and the confusions of nature and nurture, more malign than in the study of differences among human groups. Older textbooks on race sooner or later come to the question, always treated with a certain prurience, of inherited differences in intelligence. That such dillm*uces existed and that they were inborn once seemed obvious. I.imucus himself classified humans as Homo sti^icus, thinking man. For the species as a whole, he could be no more pivusi- in his definition than Homo, nosce te ipsiwr. M.in, know thyself. His description of the different varieties ot humankind, in contrast, used behaviour as a character. Linnaeus' definition of an Asian, for example, was someone who was yellow, melancholic and flexible. Even forty years ago, racial stereotypes of the most predictable kind were still the norm.

  Much of the work on inherited differences in intellect among races is contemptible and most of the rest is wrong. The wrong argument goes like this. Blacks do less well than whites on IQ tests, so that they are less intelligent. The IQ scores of parents and children are similar, so that differences in intelligence are controlled by genes. The difference between blacks and whites must therefore be set in DNA.

  This argument is deceptively simple. It was once used in the USA as an excuse not to spend money on black education, and a variant of the theory, which sees poor rather than black children as victims of their genes, is employed in Britain by those who resent investment in state education (although, oddly enough, the most devoted hereditarians improve their children's environment by sending them to private schools). Simple as it may be, the argument is utterly false.

  Whether IQ tests are an unbiased measure of intelligence is a matter for those who design them. The general consistency in the ability to move shapes around, or to do simple language puzzles and sums suggests a certain objectivity in the measure. The similarity of parents and children in their ability to do the test does not in itself say much, as families share the same environment as well as the same genes. It would be surprising if there were no genetic component in IQ variation. Many with low IQ suffer for genetic reasons, as several inborn illnesses manifest part of their effect by damaging the brain. Although normal variation in intelligence may not be related to such genes — after all, inborn blindness has nothing to do with variation in colour perception — a few inherited diseases do alter specific parts of the IQ mix. One, Williams syndrome {which involves the loss of a tiny section of chromosome), causes heart problems and a rather odd appearance; and a complete inability to deal with objects in space. Patients asked to draw a bicycle do a reasonable job with the wheels, the handlebars and the pedals — but they are scattered on the page. They find it impossible to arrange the parts into an image of the whole machine. However, their ability to speak or to do sums is not much affected.

  This rare disease suggests that separate genes affect different parts of the IQ mix but, as always when using the abnormal to study the normal, says little about variation in the population as a whole. A mass of evidence from twins and adoption does suggest an inherited component to IQ. Indeed, a variant form of one gene involved in the growth of cells is frequent among children of very high intelligence (although it explains just a small part of the total variation). The gene involved helps move enzymes around inside cells and those of high intelligence may be more effective at burning sugar in the brain. Some claim that as much as seventy per cent of the variation in score within a population is due to diversity in its genes. This figure seems high, but can be accepted for the present. At first sight it looks like powerful evidence for the view that any racial differences in IQ must be set by biology.

  In fact it has no relevance to understanding whether such differences — if they exist —;mi inborn or acquired. Why this is so can be seen in another character. In the United States, the blood pressure of middle-aged black men is about fifteen points higher than that of whites. Twin and other studies show that about half the variation in blood pressure within a group is due to genetic variation, and some genes that influence the character have been tracked down. The figures for blood pressure look similar to those for IQ although in this case blacks come out with a higher score.

  Doctors and educationists have a subtle difference in world-view when faced with such figures. Doctors are optimists. They concentrate on the environment, the fact
that blacks smoke more and have poorer diets than do whites, and try to change it. In the USA, optimism has paid off and high blood pressure among blacks is less of a problem than before. Many educationists are less hopeful. To them, the existence of inherited variation in intelligence removes the point of trying to improve matters with changes in the environment. Blacks, they say, have worse genes. These cannot be altered, so that it is futile to spend money on better schools. Their theory has been proved wrong. Over the past thirty years the average IQ of Japanese children has risen to ten points higher than that of Americans. Not even the most radical hereditarian claims that this is due to a sudden burst of evolution in the Far East. Instead, the schools are getting better.

  Both genetical and environmentalist views of blood pressure or IQ are naive. Characters like these are shaped by both gene.ind environment and it is meaningless to ask about genetic differences except in populations that live in the same conditions. I once did a simple experiment with a group of students. 1 divided them on the basis of hair colour. The fair-haired group were sent downstairs for coffee. The other set measured their own resting blood pressure. I then summoned the coffee-drinkers. As they had just run upstairs and were dosed with caffeine, their average score was higher than that of the dark-haired students. There was an association between blood pressure and hair colour.