Lactase non-persistence is an example of a genetic trait that has been influenced by cultural factors.Domestication of animals by humans began approximately 10  000 years ago. Farming of cattle provides a reliable source of meat and milk. Milk is rich in nutrients and calories, and, through dairy farming, provides more calories than would be yielded if the cows were consumed as meat. Milk from domesticated cows has been a valuable food source in parts of Europe for 8  000 years as dairy farming became an established practice.

Lactose is a disaccharide sugar found in milk. The enzyme lactase catalyses the breakdown of lactose into glucose and galactose, which are absorbed into the blood from the small intestine. If undigested, the lactose molecule is too large to be absorbed into the blood. Instead, it continues into the large intestine, where it is fermented by bacteria.

The production of lactase drops by approximately 90% during the first four years of life due to the switching off of the lactase gene. This switching off is environmentally induced by the weaning of the baby from the mother's milk. Globally, about 70% of all adults have inactive alleles and do not produce lactase, so they are unable to digest lactose. The switching off of the lactase gene results in lactase non-persistence in adults, which is the normal genetic condition.

At some stage in human evolution, a dominant mutation occurred that prevents the switching off of the lactase gene, allowing the production of lactase into adulthood. People with this mutant allele are able to digest lactose throughout their lives. This is known as lactase persistence. Evidence indicates that the mutation occurred independently in populations in northern Europe and parts of Africa before or at about the same time as cattle were domesticated

Discuss the presence and occurrence of lactase persistence in different regions of the world.In your discussion consider:

• The genetics and inheritance of the lactase persistence allele in humans
• The role of cultural evolution in the selection of lactase persistence in only certain regions of the world
• The reasons for the current frequency distribution of lactase persistence.