LCT
Lactase gene
What it does
The LCT gene produces lactase, the enzyme that breaks down lactose, the sugar in milk. In most mammals and most humans, this enzyme switches off after weaning. A regulatory variant nearby (in the MCM6 gene) keeps it switched on for life in some populations.
The variants that matter
The 'lactase persistence' variant keeps lactase production going into adulthood. It is common in populations with a long history of dairy farming and uncommon across much of South and East Asia.
If you carry the notable variant
If you lack the persistence variant, which most Indian adults do, digesting fresh milk in quantity causes bloating, gas, or discomfort. It is not an allergy, it is the default human state, and fermented dairy is much easier to handle.
Why it matters in India
Only a minority of Indian adults carry lifelong lactose tolerance, which is why dahi, chaas, and paneer, where fermentation or processing has already broken down much of the lactose, sit so much better than a glass of milk.
The honest caveat. Lactose intolerance is a spectrum, not a switch. Many people without the persistence variant still tolerate small amounts of dairy comfortably. Test your own threshold rather than cutting dairy entirely.