Twenty years have passed since the Human Genome Project mapped nearly all twenty thousand genes encoded within the three billion molecules of DNA strung out along the two intertwining strands of the double helix. ‘We face a time of dramatic change,’ commented the project’s director, Francis Collins, anticipating ‘unprecedented opportunities’ for medicine and science in the wake of this landmark achievement. At the time, it was expected that the genes involved in common illnesses (diabetes, heart disease, Alzheimer’s and so on) would soon be discovered, allowing pharmacological innovation to be reoriented towards targeted therapies. Pinpointing the genes that had gone awry in cancer cells, meanwhile, would usher in an era of personalised, ‘precision’ oncology. Since then, genetics has become the dominant discipline in biomedical research: massive studies sequencing the genomes of thousands of participants have become routine.

And yet for all the enthusiasm (and hype), the cornucopia of scientific papers and the acres of newspaper coverage, those ‘unprecedented opportunities’ have largely failed to materialise: the genes implicated in common illnesses remain elusive, and ‘gene-centred’ anti-cancer drugs are immensely costly and for the most part of limited efficacy.

Professor of philosophy James Tabery’s Tyranny of the Gene is a thoroughly illuminating account of the reasons for this discrepancy between promise and reality. He contends, as his title suggests, that genetics has come to ‘tyrannise’ medical research, marginalising potentially more fruitful lines of enquiry.

All humans, by definition, share the same complement of genes. Within the three billion human DNA molecules, the arrangement of the four chemical bases that make up the code (designated by the letters C, G, A and T) differs between individuals in approximately ten million cases. These ‘single letter differences’