If it is religion’s job to make people feel guilty, then genetics was invented to provide the excuses. What would human beings have done without the reassuring possibility that it could all be in the genes? But the thought could equally lead to grim fatalism. Attributing human qualities and behaviour to the existence of specific units of inheritance implanted in the body itself goes back to the late-19th century. This is when Darwin and, less famously, Mendel had upset, once and for all, the certitudes of Genesis, working mankind back to origins that were far more disturbing than the sin of biting into a fruit. The implications of Mendel’s work on heredity were properly picked up only in the 20th century; towards the end of its first decade, the word ‘gene’ entered the English language in its modern sense. From Mendel through Watson & Crick into the present, with a dark twist during the Third Reich, genetics has caught on in the scientific and popular imaginations. It hovers on the brink of becoming an alternative theology that could condemn as well as absolve human nature of some of its most difficult compulsions.

So what does it mean to be told now, and not for the first time, that there is an obesity gene? (The media have been flashing such a possibility, from time to time, since the turn of this century.) Should it make people resigned to being fat, and therefore joyously stop dieting or sink into incurable gloom? Or should it inspire them to invent a pair of infinitesimal tweezers with which to pluck out this gene, like an offending worm, and then concentrate on making genetically modified babies who would all grow up to wear Size Zero? Or should one just ignore all this pop-genetics and get on with managing one’s body and mind with a mixture of sinfulness and common sense? The modern world loves genetic theories of everything, particularly of things that aren’t quite ‘normal’. Homosexuality, criminal aggression, alcoholism, proneness to addiction, and now obesity. All these ‘disorders’ have been attributed, every now and then, to genes. They are all syndromes that make happy people deeply uncomfortable. Yet, towards the victims of these syndromes, they long to be fair, tolerant or just politically correct. “Don’t worry,” they long to say, “we do understand. It’s not your fault. You can’t really help it. Learn to be a good victim and be scientific about it.”

Genetic theories of why human beings are what they are walk the tightrope between damnation and responsibility. There is a great deal that is still unknown about how genes actually determine human nature and action. And such gaps in knowledge are the most powerful catalysts for the popular imagination. So whether being told that it’s all in your genes comforts or terrifies you would depend on how free you wish to be to live out what you think you might be.