Interviewee: Matt Ridley. This gene on chromosome 2 codes for the enzyme lactase. This enzyme enables infants to break down lactose, the main sugar in milk. In people who are lactose tolerant, the gene remains active throughout their lives. In most people who are lactose intolerant, the gene is turned off after infancy, making the digestion of dairy products difficult and painful. (DNAi Location: Genome > Tour > Genome spots > Chromosome 2: lactose tolerance / intolerance > Another chromosome 2 story)

The gene for lactase – a gene that enables us to digest milk. In most mammals it's switched off in adulthood – it's only needed in infancy when young animals are drinking milk. But in many human beings – not all – it remains switched on during our adulthood. The reason seems to have been that in the ancestors of those human beings they took up dairying; they actually stole the milk from animals. And that put pressure on natural selection, for them to evolve an ability to keep this gene switched on throughout life. Those who don't have this gene switched on in adulthood have trouble digesting lactose.

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