Steve Jones is Professor of Genetics at University College, London. His book The Language of the Genes, based on his 1991 BBC Reith Lectures, is a model of how wit, learning and clear-headedness can make a complex subject intelligible to a huge audience.

The language of the genes has a simple alphabet, not with twenty-six letters, but just four. These are the four different DNA bases – adenine, guanine, cytosine and thymine (A, G, C and T for short). The bases are arranged in words of three letters such as CGA or TGG. Most of the words code for different amino acids, which themselves are joined together to make proteins, the building blocks of the body.

Just how economical the language of inheritance is can be illustrated with a rather odd quotation from a book called Gadsby, written in 1939 by one Ernest Wright: ‘I am going to show you how a bunch of bright young folks did find a champion, a man with boys and girls of his own, a man of so dominating and happy individuality that youth was drawn to him as a fly to a sugar bowl.’ This sounds rather peculiar, as does the rest of the fifty-thousand word book, and it is. The quotation, and the whole book, does not contain the letter ‘e’. It is possible to write a meaningful sentence with twenty-five letters instead of twenty-six, but only just. Life manages with a mere four.

Although the inherited vocabulary is simple its message is very long. Each cell in the body contains about six feet of DNA. A useless but amusing fact is that if all the DNA in all the cells in a single human being were stretched out it would reach to the moon and back eight thousand times. There is now a scheme, the Human Genome Project, to read the whole of its three thousand million letters and to publish what may be the most boring book ever written; the equivalent of a dozen or so copies of the Encyclopaedia Britannica. There is much disagreement about how to set about reading the message and even about whether it is worth doing at all. It probably is. The Admiralty sent the Beagle to South America with Darwin on board not because they were interested in evolution but because they knew that the first step to understanding (and, with luck, controlling) the world was to make a map of it. The same is true of the genes. To make this map will be expensive – about as much as a single Trident nuclear submarine. The task will be stupefyingly tedious for those who have to do the work, but, before the end of the century, someone will publish the inherited lexicon of a human being. To be more precise, there will be a map of a sort of Mr Average – the chart is, of course, of a male – as the information will come from short bits of DNA from dozens of different people …

Human genetics was for most of its history more or less restricted to studying pedigrees which stood out because they contained an abnormality. This limited its ability to trace patterns of descent to those few families – like the Hapsburgs – who appeared to deviate from the perfect form. Biology has now shown that this perfect form does not exist. Instead there is a huge amount of inherited variation. Thousands of inherited characters – perfectly normal diversity, not diseases – distinguish each person. There is so much variety that everyone alive today is different, not only from everyone else, but from everyone who ever has lived or ever will live. The mass of diversity can be used to look at patterns of shared ancestry in any family, aristocratic or plebeian; healthy or ill. Because all modern genes are copies of those in earlier generations each can be used as a message from the past. They bring clues from the beginnings of humanity more than a hundred thousand years ago and from the origin of life three thousand million years before that …

There have been claims that we may soon find the gene that makes us human. The ancestral message will then at last allow us to understand what we really are. The idea seems to me ridiculous.

Just how ridiculous it is can be seen by looking at the search for another important gene, one which I inherited from my father, and he from his and so on back to a distant ancestor that lived long before the birth of our own species. This is the gene that makes me male. The maleness gene was tracked down recently and its message spelt out in the four DNA letters, A, G, C and T. It starts like this: GAT AGA GTG AAG CGA. There are 240 of these letters altogether and, between them, they contain the whole tedious biological story of being a man. This brief ancestral bulletin does nothing to tell that half of the population which is unfortunate enough not to have it what it is really like to be male rather than female. Being a man involves a lot more than a sequence of DNA bases; and the same is true for being a human.