Steve Jones - The Language of the Genes

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 sound 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.