Garth writes in asking about rates of lactose intolerance around the world. James Tytko asked Mark Thomas, Professor of Evolutionary Genetics at UCL, to help with the answer...

James - Thanks Garth. Lactose is a sugar found in milk. The enzyme responsible for breaking down lactose into more absorbable sugars is called lactase, and it’s something we make a lot of as babies when, for the most part, our diets are significantly constituted of our mothers’ milk.