In affected people, the production of the enzyme declines during or shortly after childhood, resulting in lactose malabsorption. Undigested lactose sugars affect the development of gut microflora, leading to dysbiosis. Two copies, or alleles, of the lactase gene are present in the genome. Each allele can be either type T (normal) or type C (abnormal).
A patient can have three possible genotypes:
- T/T: normal expression of the enzyme, patients are lactose tolerant
- C/T: only one functional copy which is sufficient for lactose metabolism, therefore these patients can be considered lactose tolerant
- C/C: these patients are lactase deficient, their tolerance to lactose is limited