Lactose intolerance results when a person inherits two defective alleles (alternative forms) of the lactase gene. Normally, this gene produces the enzyme lactase, which breaks down lactose during digestion. What would most likely happen if a person had one defective allele and the other allele was functional?

The condition of lactose intolerance is usually produced by the inheritance of recessive genes that sport a mutation in them which affects the production of lactase, the enzyme that breaks the sugar present in milk, lactose. When two inherited genes with this mutation happen in a person, they will develop the condition that is also known as congenital lactase deficiency. This means the body will not produce the enzyme to break up the bonds present in milk's sugar.

However, sometimes, this condition can be inherited from one of the parents, and not from both, which would make one of the alleles fully functional and the other defective. The result would be that this person would still be able to produce lactase, but in much smaller amounts, so they would still have some of the symptoms and signs characteristics of lactose intolerance, but they would be able to digest at least a portion of the lactose present in milk and dairy products.