Some may be less fortunate. The Techa River tuns through Chelyabinsk, once the nuclear capital of Russia. During the height of the Cold War, so much waste was dumped that a fisherman on its banks could get a lethal dose in a week. Many riverside villages have been moved, but the inhabitants of those that remain have increased levels of leukaemia and of cancers of the thyroid and other organs. Even so, the effects are less than those suffered by the survivors of the Hiroshima bomb, so that a sudden burst of radiation may be more dangerous than the same dose given over a longer period. As western safety standards are based on the Japanese cancer figures, there have been calls to relax the minimum dose, but those are resisted by many who believe that no safe lower limit exists.
Whatever the effects of radiation on cancer, chemicals are more important. Those in tobacco smoke are potent agents and some industrial chemicals are just as bad. Alcohol, too, is far from blameless. Certain chemicals bind to DNA to cause their damage. The amount of bound material gives an estimate of exposure to mutagens. The Polish city of Gliwice, which burns much soft coal, is one of the most polluted places in the world. Gliwice has a high rate of cancer. Many inhabitants have large amounts of poisonous chemicals stuck to their DNA. The amount goes up in the winter, when the smoke is at its worst. Many of those exposed will develop the disease.
Other cancers (such as retinoMastoma, a degenerative disease of the retina) run in families, with no obvious environmental link. The causes of the disease run all the way from gene (predominant in retiiioblasioiu.i) to environment (equally so in scrotal cancer), but usually includes both. Workers in the primitive oil industry believed that people with fair hair and freckles should not be employed they were at risk of 'sootwort', as scrotal cancer was known. As such people are in more danger of skin cancer when exposed to sunlight, there may be some truth in the idea.
Seven million die of cancer each year. Many expose themselves to environments so dangerous that even the finest genes cannot save them. Caliban himself could not have devised a contrivance as fiendish as the cigarette: a cheap drug delivery system that provides a narcotic as addictive as heroin and some of the most carcinogenic of all chemicals. Hundreds of millions of people have volunteered themselves as subjects in a gigantic experiment and millions have obligingly died. Their generosity proves the joint actions of nature and nurture. If everybody smoked, lung cancer would be a genetic disease. Many of the cellular checks and balances have a lot of natural variation. For one crucial member, about one person in ten has a highly active form which, when faced with tobacco smoke, does its job very well — and, as it does so, produces a dangerous carcinogen. As a result, light smokers with this form of the gene face a seven times greater risk of lung than do those with other variants (although in heavy smokers, who batter all their defences into submission with massive doses of poison, the risk merely doubles).
Even when DNA is damaged, it can be repaired. A few families lack one or other of the enzyme systems involved,uid as a result are at high risk. Their activity in the population as a whole varies a hundredfold; and, once again, those with the feebler forms are at increased risk if they smoke. Indeed, such forms are five times commoner among smokers with lung cancer than in those who escape the disease. Blacks who smoke have higher rates than do whites, and this too is associated with some unidentified genetic difference among the groups.
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.