Mark Henderson, science editor of this newspaper, recently unveiled his genotype to the nation. His risks of developing type 2 diabetes and colon cancer are no greater than average; his genes lower his chances of becoming obese or developing Alzheimer’s disease.

Divining one’s future physical decline using DNA is one thing, but gene testing is now straining at an even more controversial frontier. A Californian company, Psynomics, recently – and rather discreetly – started marketing a test for bipolar disorder (previously called manic depression). Another company plans to launch a similar test for schizophrenia; yet another plans to predict whether you will develop suicidal thoughts while on antidepressants.

Before you sign up for the Psynomics test, consider the very nebulous science that lies behind it. There are scores, perhaps hundreds, of genes implicated in bipolar disorder, each imparting a small fraction of risk. But they are thought to be switched on by as yet unidentified environmental triggers. Even if you were in possession of the most damning set of gene variants, you’d have only a 3 per cent risk of developing bipolar disorder – or a 97 per cent chance of never developing it. More damagingly, some genes that appear to contribute to the condition in some studies don’t show up in others. Nobody knows with any certainty which genes underlie it.

John Kelsoe, of the University of California, the geneticist behind Psynomics, is pinning his hopes on a gene called GRK3. But, according to Science, other academics feel its contribution might be negligible. While Dr Kelsoe found a bunch of incriminating DNA changes in the GRK3 gene in his bipolar patients, David Collier, of the Institute of Psychiatry in London, failed to uncover them in the 410 bipolar patients he examined. Dr Kelsoe admits that his hunch about GRK3 might be baseless.

Kurt May, CEO of Psynomics, has bristled that “we . . . don’t have to win the Nobel prize on this one . . . All we have to do is to present a business model that is one better than psychiatry today.” That’s the thing: psychiatry is nowhere near telling us which genes mess with your mind, which is worth remembering before posting a cheque for $750 to California.

The Prime Minister’s refusal of a genuinely free vote on aspects of the Human Fertilisation and Embryology Bill is misguided. Pushing the Bill through will, he hopes, position Britain at the forefront of embryo and stem-cell research. But at what cost? Even a progressive such as me recognises that abortion time limits, human-animal embryos and gay parenthood deserve to be treated as issues of conscience.

To insist otherwise risks breeding ill-will among a public that has cautiously and thoughtfully supported progress in this nascent field. That public expects to see MPs casting their votes on the basis of conscience and considered reason, not coercion.

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