Others find that their lactase production gradually slows or even stops as they grow older. This is controlled by a gene on the second of the 46 pairs of chromosomes that humans have. One set of each pair of genes comes from our fathers and one set from our mothers. The go gene is dominant and the stop gene is recessive. That means that if you have a stop and a go, go always wins.
Lactose tolerance is genetically dominant.
When you stop to think about it, this has to be true. Today over a billion people are not. For a trait to spread across the world that quickly, it must obviously be dominant, so that it will be transmitted to children even if only one of the parents has it. If nobody drinks milk as adults, the trait sits there quietly and does nothing. With this as background, let's go back to your case.