People who are lactose intolerant cannot digest lactose, the primary sugar found in milk and dairy products. Lactose intolerant individuals suffer bloating, gas and other intestinal problems when they ingest foods containing lactose. Lactose intolerance during adulthood is the norm for the majority of people around the world, but some population groups have developed mutations that allow them to process the sugar in milk past babyhood.
Lactose Intolerance
Lactose intolerance develops when the body slowly slows, and sometimes completely halts, production of the enzyme lactase. Lactase is normally produced in the walls of the small intestines during childhood and is used to help digest the sugar lactose, which is present in human breast milk. After weaning, the biological need for lactase decreases and production slows. Since lactose is also present in cow's milk and other dairy products, as individuals develop lactose intolerance they become unable to consume these products. Lactose intolerance in adulthood appears to be the genetic norm and most people in the world lose their ability to produce lactase over time.
Associated Genes
The LCT gene on chromosome 2 carries instructions for making the lactase enzyme. However, another nearby gene called MCM6 is responsible for maintaining the ability to produce lactase. MCM6 produces a protein complex that attaches to the LCT gene and allows it to produce lactase. In many people, MCM6 activity decreases over time, which causes a decrease in lactase. Four different mutations have been found that change MCM6 and cause it to continue acting on LCT long after childhood. People with these mutations are able to consume products containing lactose since their bodies still produce lactase in adulthood. Mutations in the LCT gene can also occur, which can result in lactose intolerance from birth. This is a dangerous condition for affected babies, since they cannot consume breast milk or formulas containing lactose.
Inheritance
Babies who inherit a congenital lactose intolerance must get a mutated gene from both parents, since even one good gene will allow enough lactase production to properly digest milk. Normal lactose intolerance, caused by the decreased activity of the MCM6 gene, occurs in about 65 percent of people worldwide, so it is actually the normal condition, not a mutation. In fact, to be able to produce lactase in adulthood, you need to inherit one or more of the mutations in the MCM6 gene.
Population Genetics
The MCM6 mutations allowing for sustained lactase production are present in about 95 percent of people of Northern European descent. Some communities in India and the Tutsis in central Africa also have high levels of the mutations that increase lactase production. In contrast, only 10 percent of people from East Asia have one of these genes, leaving 90 percent of the population lactose intolerant in some regions. West African, Jewish, Greek, Arab and Italian people also have low incidence of the MCM6 mutations, so many people with these ancestries are lactose intolerant.
