Definition
Lactase persistence is the genetic trait that allows adult humans to continue producing the enzyme lactase, which digests milk sugar (lactose). In most mammals — and in most humans worldwide — lactase production declines after weaning, and adults cannot efficiently digest fresh milk. Lactase persistence is the derived condition: lactase production continues into adulthood, enabling lifelong consumption of fresh dairy.
The trait is controlled by regulatory variants near the LCT gene that keep lactase transcription switched on. Multiple distinct variants have evolved independently — one in northern Europeans, several in different African and Middle Eastern dairying populations — making lactase persistence a textbook example of convergent evolution within our species.
Why it matters
How it works
In humans without the persistence variant, the LCT gene is transcriptionally silenced after weaning, lactase production drops, and adult lactose consumption causes gastrointestinal symptoms. The persistence variants disrupt this silencing — typically by altering regulatory elements upstream of LCT — so that transcription continues throughout adulthood.
In populations that adopted dairying, adults able to drink milk gained a substantial nutritional, caloric, and fluid advantage, especially in environments where alternative resources were unreliable. Selection for the persistence variants was sufficiently strong to drive them to high frequencies in a few thousand years. Today the European variant -13910*T sits near 70 percent in northern Europe, declining southward across the continent.