Why are some people lactose intolerant?

My dad recently found out he's lactose intolerant. Whenever he has milk or anything with cream in it, he has what he calls a 'major arse explosion'. I don't think his arse actually explodes, but the smell coming from the loo is always pretty bad. And when he's done, he's usually pretty exhausted.

My mom thinks he's been lactose intolerant for years and just didn't realize. My dad was not too bothered about his angry farts. But my mom got pretty upset with him at some point and that's why she put him on a non-dairy diet. For two weeks, he could only eat stuff without any milk or cream in it; she didn't even allow him to have milk chocolate. And when he finally started drinking milk and eating cheese again last Saturday, things became really, really bad...

My mom's not lactose intolerant. She can have whatever she wants, normally it's lots of creamy cakes and nougat chocolates. I know no one in school with lactose problems, and I am not intolerant either. So I was wondering why only some people have 'arse explosions' and others don't.
 
I found out that lactose is a special kind of sugar that only occurs in dairy products. (If you're into chemistry, you might like to know what it looks like, so check out this picture of its full chemical structure on Wikipedia). Lactose is a part of all kinds of milk produced by mammals. This means that it's also found in human breast milk.

The real problem with it is that it needs to be broken down into glucose by our bodies. And this can only be done by a special enzyme called Lactase. Lactase is normally produced in our small intestines (that's where the first part of digestion happens). It breaks down lactose into glucose so that our body can absorb it via the bloodstream.

If you can't produce lactase, lactose apparently goes right through you and ends up in your colon. Our colon can't absorb the lactose without lactase. So it just starts working on it with the bacteria it has. And this can become very messy because it will produce huge amounts of acids, carbon dioxide, hydrogen and methane.

Anyway. The NHS has a very good site on lactose intolerance. You can check it out here. It says that there are two different types. One is called 'primary lactase deficiency', the other one is 'secondary lactase deficiency'. The first one is inherited, the second one is what you have if there is something wrong with your intestines. Most people inherit lactose deficiencies from their parents though. I found out that it can sometimes take really long until you notice it. Which would explain why my dad thought he was normal, although he smelled like a monster every time he let one rip.

Babies are immune because they produce huge amounts of lactase throughout the first two years of their lives (unless they have a problem with breast milk right away). But the gene responsible for lactose intolerance (the LCT gene) is usually passed on by parents who have at least one mutation of a gene called MCM6. (Check out this governmental website on genetics to learn more about genetic mutations in relation to lactose intolerance).

What this means is that -  although only one of my parents is actually lactose intolerant - it might still be enough for me to get some major arse explosions in the future...!

I took the bomb picture from fdwallpapers.com. The belly and the cream came from an Indian website called www.medindia.net/. Grover and his smell came from fanpop.com.