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LI and Genetics

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Which of these two statements is true? A) Lactose Intolerance (LI) runs in families. B) You can be LI even if neither of your parents are.

Oddly enough, they're both perfectly true. Here's why.

As recently as 10 thousand years ago, virtually every single person in the world was LI. They could all drink milk as children, of course, but sometime after weaning their intestines stopped producing an enzyme called lactase that digests the lactose in milk. The small intestine stopped doing this because a certain gene on chromosome number 2 sent out a signal that told it to do so. Let's call this gene "m".

Genes are coded by DNA and usually our bodies are extremely good at making exactly perfect duplicates of our DNA. Every once in a while, though, our bodies slip up and make a mistake. These mistakes are called mutations. Some are harmful, some have no effect whatsoever. A few create changes that are actually useful.

Just by chance, then, a few people must have had a mutation in the m gene that changed it so that the gene never sent the small intestine the stop lactase production signal. Let's call the mutated gene "M". Was this change beneficial or harmful? At first, it was neither. Unless you were to drink fresh milk as an adult, you'd never notice whether you had the m or M version of the gene.

But in northern Europe, there were few good sources of calcium. And the colder weather allowed the farmers there to drink their milk fresh instead of processing it into low-lactose forms like butter and cheese. And so the M gene adults could drink milk without suffering from the symptoms of LI.

M gene people who make lactase can process the calcium in milk better than m gene people who are LI. Calcium was especially important in the days before medical care. Those who had more calcium in their bones were healthier and stronger. Women were more likely to live through childbirth. The M gene gave them a very slight but crucial genetic edge that spread the M gene through the population. This is why people of northern European descent alone are likely to be able to drink milk as adults while most people of other heritages cannot.

One other factor has to be considered, though. The M gene is a dominant gene. If a child inherits it from either parent, that child will be an adult milk drinker. The m gene is recessive, so the child has to get it from both parents to become LI. This is why both the statements up top are true. Take LI first. Children who get an m gene from both parents will naturally become LI. So LI obviously runs in families. But so does milk drinking. Children who get an M gene from both parents can drink milk all their lives.

But it gets tricky when the genes are mixed. Say that we have a man and a woman who both have inherited the M gene from one parent and the m gene from the other. Both therefore have an Mm combination, and so neither will be LI. If they marry, three things can happen. Their children can be MM, and drink milk. Their children can be Mm, which also means that they can drink milk. But - and here's the tricky part - their children might, just by chance again, wind up mm, If so, they will be LI even if neither of their parents were.

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