Height Hopes

A new study identifies potential height genes
Height is a confusing trait genetically. And a recent study only clears things up a bit. The study identifies three or four regions in our DNA that may be important for height. But it doesn't find any specific gene or DNA change responsible for men being taller than women. Or any gene to explain why height can run in families. Or anything else specific. Well why not? Part of the reason is that the studies are still at an early stage. You tend to narrow down the important regions over time. So lots of studies start out with a few regions at first. But scientists have been studying height for a very long time. And they haven't gotten any closer than this. It isn't like scientists can't find genes for other traits. They have found the gene for blue eyes. And the one for red hair. But height is different for a couple of reasons. First, there is almost certainly not a single gene involved. There may be as many as 20. This makes it much harder to trace a family tree and find the genes responsible. The other tricky part is that the environment can have such a big effect on height. Look at the Japanese, for example. Overall, they used to be much shorter than Northern Europeans. Now they are as tall. Why? Because they get more protein than their ancestors did. In other words, their new diet lets them live up to their genetic potential. This combination of lots of genes and huge environmental effects makes height very difficult to study. Imagine you have a family where you have kids of different heights. The first born is shorter than the later born child. Is it because of differences in their genes? Or differences in their environment? Maybe mom didn't eat right when she was pregnant with her first born. And this is why the first born is shorter. Or maybe mom smoked when she was pregnant with her first child. Or she didn't have good medical care. Or… As you can see, it can be pretty tricky to figure out what part of height is due to genes. And what part is due to the environment. This is one reason that this study is significant. They have tried to minimize the effects of the environment by looking at fraternal twin pairs. And maximize the chances of finding common genes by looking at lots of these twin pairs.
Why fraternal twins?
To try to minimize the effects of environment, the researchers looked at fraternal twins. As you know, there are two types of twins—fraternal and identical. Identical twins come from the same fertilized egg so have exactly the same DNA. Fraternal twins come from different fertilized eggs and so have DNA that is as similar as any brother and/or sister. What we usually hear about are studies using identical twins. This is because identical twins are so useful for figuring out how much the environment can affect a trait. We know that if one twin in an identical pair has blue eyes, then the other almost always does too. This tells us that the environment doesn't really influence eye color much. Because these twins have exactly the same DNA. Now imagine that one twin develops Type 1 diabetes. The other twin has less than a 50% chance of developing it as well. So the environment has to play a big role because, again, the twins have the exact same genes. So eye color is mostly genetic and diabetes is partly genetic and partly environmental. These kinds of studies won't work with fraternal twins because they don't have the same DNA as each other. But where fraternal twins can come in handy is when you are trying to figure out the genetics of something that is heavily influenced by the environment. Fraternal twins have a pretty similar environment in the womb and usually as they are growing up. Now imagine that two twins in a pair are different heights. Part of this may come from different environmental effects (like the diabetes example above). But these twins will have the closest possible environments of any two people with different genes. They will certainly have a more similar environment than siblings who come before or after them. This is why fraternal twins are so useful in looking for genes that are heavily influenced by the environment. The environmental effects are minimized as much as possible. So researchers can get a good look at the genes involved.

More Information

Genetics of height
Example of an identical twin study
Fraternal twins like these
can help us find
height genes.
What they learned about height

Height genes may be on
the X chromosome and
chromosomes 7, 8, and 20.
OK, so what did they learn? Like I said, they didn't learn why someone is tall and someone else isn't. Or why men are usually taller than women. But they did take us a step closer to figuring out at least the first part. Because height is so complicated, these kinds of studies tend to give conflicting results. One study will come up with one part of the DNA and another study will identify a different part. Or the environmental effects will blur everything so that the researchers can't tell anything. As I talked about, one thing the researchers did to try to get around this was to use fraternal twins in their studies. The other thing they did was look at a lot of different sets of twins. In fact, they looked at 3,817 different families. Large numbers help find common genes when lots are involved. Imagine there are ten genes involved in height. If you look at ten pairs of twins, their differences may each be due to a different gene. So when you look at the results, nothing stands out. But if you look at 1000 pairs of twins, now you can begin to group them and look for patterns. These 100 are different heights because of one gene and this other 100 is a different height because of another gene. By looking at so many pairs of fraternal twins, the researchers found the most significant DNA region for height to date on chromosome 8. They also found a less significant region on each of the following chromosomes: 7, 20, and the X. The region on chromosome 8 has been found by other researchers too so this is probably a key region. Scientists can now hone in on this region and find the key gene(s) involved. But the researchers did not find a region to explain the differences between males and females. This may not be all that surprising as perhaps there are many genes that contribute to this difference. For example, what if testosterone affects many or all of the different height genes? Then it would be very hard to find a male specific height gene because there are actually lots of them. Or maybe the researchers just didn't have enough boy-girl twin pairs to look at to figure this out. We'll have to wait for them to look at more twins to try to figure this out.

More Information

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