Around 75% of the world’s adults can’t drink a lot of milk. And yet most could as a child. What’s going on here?⁠ As you can guess from the numbers, being lactose intolerant is normal for an adult. People who can drink milk as adults have a DNA mutation. In fact, being able to drink milk as an adult is now referred to as a condition called lactase persistence. What most adults have trouble with is the sugar in milk, called lactose. When we’re children, almost all of us make an enzyme called lactase. ⁠ ⁠ Lactase is an enzyme that breaks down lactose into sugars our body can use. And children make lactase because they have a working lactase gene. But then, over time, most of them lose that ability. Some lose it as early as 3, some at 20, some even older, and some never at all. If they have the gene when they’re young, where does it go when they get older?⁠ ⁠ _____⁠ ⁠ It doesn’t go anywhere; it just gets shut off. Remember, for a gene to work, it must be read by the cell and translated in...