Lactose intolerance is an autosomal recessive trait. The allele variations differ in the temporal aspect of their expression. The dominant allele is expressed later and, therefore, the phenotype for the dominant allele is the persistence of production of the lactase enzyme. The recessive allele is expressed only early in life and results in lactose intolerance. The gene that codes for the lactase enzyme is located on chromosome 2. This is the case in all populations, but the reguation of the gene varies. The lactase gene has four common haplotypes- A, B, C, and U. The A haplotype is usually found in populations that exhibit lactase persistence. Increased haplotype diversity occurs in populations that exhibit lactose intolerance. The expression of the lactase persistence phenotype gives genetic advantage to members of populations that consume milk. This is why most express the dominant allele in milk consuming populations. While it is still unclear, it is likely that lactase regulation occurs during DNA transcription because there is variation in mRNA levels between lactase persistent and impersistent individuals.  Genetic drift probably also played a role in latase gene diversity. The more homogeneous populations are probably as such because of genetic drift, this is why African cultures have more diversity in the haplotypes. Gene flow, mainly due to colonization, is probably the cause of continued frequencies of lactose intolerance and tolerance within populations. Lactose tolerance is rare in a worldwide sense, but it exists in scattered populations that have no contact. As shown by the low levels of lactose intolerance in United States Whites and the Fulani and Tutsi tribes, the genes evolved independently in different populations around the world.