People who are lactose intolerant cannot digest and absorb the milk sugar lactose, because they produce only low levels of the lactase enzyme. This causes

The likelihood of lactose intolerance depends on ethnicity. Lactose intolerance is the normal phenotype.  The mutations associated with lactase persistence occurred, and were selected for,

The MCM6 gene encodes a protein that controls the production of other proteins. This includes lactase, encoded by the nearby LCT gene. People with at least one of these

Lactose intolerance causes undigested lactose to build up in the large intestine, resulting in the growth of gas-producing gut bacteria. Symptoms include: