"We have to recognise that the real enemy is the damn disease," he said. "You shouldn't assume that mistakes don't occur within the genes and that they don't affect the body's function after you're born. But for some reason this is a red rag for some people. They say that you shouldn't try to test for them. It's absurd. Parents aren't going to abort a child just because it's got the wrong colour eyes. But maybe they will if it's only ever going to have the mental age of a five-year-old. Or if it's going to be a Down's baby. At least with testing you have the choice . . . would you want [to have] a child you knew would develop schizophrenia?"
Dr Watson, who works at the Cold Spring Harbor Laboratory, in New York, but was in London for a conference on gene theory, said prohibiting companies from using genetic testing would not end discrimination. "If somebody applies for a job who weighs 400 pounds, or who has a misshapen face, a company is less likely to hire them. The real discrimination comes from disease . . . if you develop motor neurone disease your working life is shortened."
Humans have an estimated 50,000 to 100,000 genes, arranged on 23 pairs of chromosomes. Each gene consists of millions of "base pairs" of four amino acids, arranged in the twisting "double helix" of DNA - the structure that Dr Watson and Francis Crick discovered at Cambridge University in 1953.
Dr Watson, 67, was the first director of the international Human Genome Project, which aims to find the sequence of 35 billion pairs of amino acids comprising human DNA and has been running for seven years.
Opponents of the sequencing programme say that genes are being linked not only to disease but to behaviour as well, including traits such as violent tendencies and homosexuality.
But Dr Watson defends the value of this research, too. He describes a study of a Dutch family which found their tendency towards violence derived from their lacking a gene which creates an enzyme that breaks down chemicals produced when someone becomes angry.
He also supports the principle that allows the patenting of the sequences of human genes, in order to exploit them commercially by developing diagnostic tests for them: "You have to look at what system works best for improving the quality of human life. Things get done better and faster if people make money in the process, I'm afraid. And patents only last for 17 years or so . . . and then the information will be available to everyone for free."
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