It is easier to say what has not been discovered: the researchers in the United States whose findings were announced last week have not found a gene that causes homosexuality and they have not proved that homosexuality is hereditary.
They believe they have evidence linking a region of the X chromosome - which men inherit from their mothers - with the sexual orientation of some gay men. If they are right, they have uncovered hard evidence of a genetic basis of homosexuality.
The gene has still to be found. The specified region of the X chromosome may carry several hundred genes in all, and the researchers are still some way from identifying which one it is.
The scientists also emphasise that their discovery does not account for every cause of homosexuality, which they acknowledge is the result of a complex interaction of environmental factors, such as upbringing,and genetics.
Why have the scientists been doing this research?
The researchers are based at the National Cancer Institute near Washington DC, which has been involved in the fight against Aids (see below). Dean Hamer, the head of the research team, said the study was part of the institute's efforts to identify genetic factors involved in cancers that are frequently found in gay men infected with the Aids virus.
What advances led to this discovery?
There are essentially two reasons why such research is now possible. One is to do with Aids, which has resulted in many more doctors recording a person's sexual orientation on medical notes, creating for the first time a large bank of statistics on these matters. This has allowed scientists to identify homosexual men more easily and compare them with heterosexual men of a similar age and background for any differences that may be linked with their differences in sexuality. It is now possible to find enough gay men with gay relatives, such as brothers or cousins, to see if they have anything in common, such as genetic similarities, that could be linked with or even account for their homosexuality.
The second advance is in the field of genetics, where it is now possible to study genes, and more precisely the DNA, the chemical blueprint of life that is the stuff of genes. Scientists can now locate the precise position of a gene and find out what it is responsible for, and what variations of that gene exist.
So what is a gene and how can it influence sexuality?
There are about 100,000 genes in humans, each responsible for the information needed to make a protein. In other words, genes provide the recipes for proteins. Some of these proteins form the structural material of our bodies, our flesh, blood and bone. Most are in fact enzymes, which control the millions of complex chemical reactions that take place in each cell of our bodies. Many of our genes - perhaps most of them - may actually be responsible for enzymes that work in our brains.
It is possible that the gene Dr Hamer is looking for works in the brain and has some influence in that way on sexual behaviour. A version of the gene in some men may therefore make them more prone to being attracted to the same sex. But the advance announced by Dr Hamer is not so clear or detailed as this. We do not yet know what this gene does, and until we do its role remains a matter of speculation.
Does everyone have this gene?
It would appear we do - even women. It resides on the X chromosome, which men have just one copy of, but women have two. What seems likely is that some homosexual men may have a different version of it (an allele, in genetic jargon) and it is this version that can give rise to the homosexual predisposition.
How do homosexuals feel about all this?
The reaction has been mixed, with the split occurring largely between gays in the United States and gays in Europe.
American homosexuals have largely welcomed the findings of a genetic basis for their same-sex preference. They believe it shows their sexuality is not the result of a free-will decision, but comes out of an act of nature. It shows their sexuality is little different from other traits with a genetic component, such as height, eye colour or race. In short, they welcome it because they believe it indicates that homosexuality is natural.
Gay activists in Britain, however, are suspicious of any scientists who make a study of homosexuality. They fear they are being singled out and labelled as a problem because of a perceived abnormality. They fear that homophobic elements in society will use the genetic research to 'diagnose' or 'treat' homosexuality as if it is a disorder to be cured or eradicated.
What is the record of science on homosexuality?
It is little wonder that some homosexuals are suspicious. Scientists have had quite a lot to say about homosexuality in the past, and most of it has been nonsense. In the 1930s, the age of the eugenic movement when it was argued that a better, fitter, more intelligent human race could be produced by genetically-guided breeding, scientists believed homosexuality was a genetic defect. This notion was used by the Nazis to justify a campaign of extermination. Elsewhere, it gave support to prejudice and to a view that homosexuals were inferior.
Generally, however, the study of the scientific basis of homosexuality has been the preserve of psychology and sociology. The classic Freudian view is that the emotional development of children is the result of parental influences. Over-protective mothers and fathers who are weak or distant were perceived to cause a young boy to stray from the path of 'normal' heterosexual development. Most of this, however, has been unverifiable conjecture. In recent decades, studies of twins and brothers who are gay have shown that there is a genetic component to homosexuality.
Does this have implications for Aids?
Not directly. If there are subtle genetic differences between gays and non-gays it might result in differences in the way people suffer from the effects of the Aids virus. It has long been noted, for instance, that gay men with Aids are more likely to develop Kaposi's sarcoma, a cancer of the skin and connective tissue.
Why has the gene not died out?
Fewer homosexuals reproduce, so by the laws of mathematics and evolution one would normally expect the gene to disappear over time. But anyone who tries to explain why it has not is quickly drawn into wild speculation.
Richard Dawkins, an Oxford biologist, says homosexuality could survive for a number of reasons. One idea is that homosexual males in early human societies may have had access to the females in a harem of a dominant male, who believed, wrongly, that a homosexual male could be trusted not to have sex with his wives. As Dr Dawkins points out, 'homosexual males do sometimes copulate with women' and such a situation would have ensured the survival of the gene 'for' homosexuality.
His other explanation is more subtle but essentially proposes that any gene that gives rise to a homosexual tendency today may not have done so many thousands of years back in our evolutionary past, when our environmental upbringing was so very different. 'Another way to put it is that the gene wasn't always a gene 'for homosexuality'. When it was subject to natural selection, it might have been a gene 'for' something quite different.'
Is there a gene for lesbianism?
There is strong evidence that there is a genetic component in same-sex attraction in women. Lesbianism does seem to run in some families, but again this may be due to similar upbringing rather than common genes 'for' lesbianism. Dr Hamer and colleagues have now begun a study of lesbians to see whether they can find the precise genetic component, if one exists.
Could science prevent the birth of gay children?
In theory, yes. If the research is verified by other scientists and is refined so that the specific stretch of DNA can be precisely identified, then it would be possible to test foetuses in their mother's womb and, if the characteristic piece of DNA showed up, give the parents the option of aborting the pregnancy.
In effect, this would be a moral judgement: the person who made such a decision would be saying that it is better for a child not to be born at all than to risk it becoming a homosexual. If the practice were to be permitted, it would cause parents immense personal trauma and anguish.
But what would happen to the child if its parents decided not to abort after a 'positive' test? It might grow up knowing that it had been labelled as genetically gay, a burden of knowledge that would inescapably affect his future life and choice of sexual partner. It could become a self- fulfilling prophecy. By themselves, these considerations would be sufficient justification for banning such pre-natal testing.
There is a further reason, and a very practical one, for preventing such genetic tinkering. Scientists know very little about how genes evolved or what evolutionary purpose they serve. Some genes do give rise to what is clearly an inherited disease, such as sickle cell anaemia. It might at first seem tempting to try to eliminate this gene, but it turns out that its existence also protects many people against malaria. What other functions might a gene 'for' homosexuality perform? We have no idea.
It would be folly to start trying to extirpate genes from human DNA while we are still so profoundly ignorant about their function and their purpose.
If you have the gene will you automatically be gay?
No. It is a myth that just because something is 'in your genes' your fate is immutably predetermined. People are complicated: they are the product of a subtle interaction of genes and environment. There is a spectrum of variation in human sexual behaviour from celibacy through monogamy (heterosexual and homosexual) to prodigious promiscuity. It is absurd to think that 'the iron hand of the genes' controls all our actions like a puppeteer pulling the strings of a doll.
Could you have it and be a happy heterosexual?
Yes. Genes, as we have seen, contain a sort of recipe for the cells in our bodies to make proteins. Proteins range from haemoglobin, which gives our blood its distinctive red colour, through to chemical messengers, (hormones), such as insulin or oestrogen. It is common knowledge that the hormones circulating in our blood affect our mood and physiology - the familiar rush of adrenalin that quickens the heartbeat, for example. But the outward expression of such physiological responses is socially conditioned. In evolutionary terms, the rush of adrenalin developed to help our distant ancestors living in the wild to run away from their predators or to turn and fight. But if you, tens of millions of years later, are having an argument with your boss, neither of these is appropriate. Very few employees ever do take a swing at their boss.
In any case, it is unlikely in the extreme that only one gene (and protein) is involved: there will be many, some adding their effect to the others, some inhibiting. There will be a chain of actions and consequences at the level of biochemistry before there is a final expression in mood or behaviour. The effect of the genetic connection whose discovery was announced last week may well be counterbalanced by other factors not yet known to science.
So is it nature or nuture?
There are conditions that are 100 per cent genetically predetermined but where the consequences of the genes can be completely negated by the effects of the environment. Some children are born with a genetic defect known as PKU, which leads to severe mental retardation if not treated. But almost all of these children grow up into normal adults with a normal degree of intelligence. Their environment has been modified: their parents make sure the childrendo not eat some foods, which are innocuous to the rest of us but which contain a chemical damaging to the children with PKU. The genes never get the chance to show their effect.
Does this mean being gay is a disease?
No. This discovery merely means that the variation in human behaviour mirrors, to some degree, variation in human DNA. From a scientific point of view, variation is vital to the survival of the human species - the dangers of 'inbreeding', where variation has been suppressed, are widely known.
Some variation is completely determined by the genes: whether someone has blue eyes or brown, for example. But no one suggests that having brown eyes is a disease or that being blue-eyed has greater moral value. Discovering that homosexuality may have a genetic component tells us nothing about the moral or social status of homosexuality. If we choose to classify homosexuality as a disease now, we have made a moral choice: it does not follow from the science of molecular genetics. There are no values to be discovered in the double helix of DNA.
The moral and social consequences of the new human genetics are explored in 'Perilous Knowledge' by Tom Wilkie, published last month by Faber and Faber, pounds 14.99.
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