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I have always wondered about the genetics of handedness. Both of my parents are left-handed, both myself and my brother are right-handed. My husband and I are both right-handed and we have 2 children, one's a lefty and one's a righty. My husband's parents are both righties. It's just something I've always wondered about since we studied genetics in 6th grade and the teacher told us that handedness was Mendelian like eye color, but couldn't explain why I'm a righty (no doubt wondering if maybe I was adopted, which I wasn't, lol!)

-A curious adult from Florida

June 3, 2005

Your teacher was close in that handedness is like eye color. Except that what makes them similar is that neither is a simple Mendelian trait.

A simple Mendelian trait is one where there is a dominant and a recessive gene. Remember, we have 2 copies of most of our genes -- one from mom and one from dad.

If some version of a gene is dominant, then you only need one copy of that version to see a trait. For example, let's say that the right hand gene was dominant over the left hand gene.

You'd be right-handed with either one or two copies of the right hand gene. To be left-handed, both copies would have to be the left hand gene.

So if two lefties had a baby, the baby should turn out left-handed. This is not the case for your family or lots of others either.

When people have looked at handedness, it just doesn't follow simple rules. For example, if both parents are right-handed, there is a 1 in 10 chance of having a left-handed child. The same is true if the dad is left-handed. If the mother is left-handed, the odds rise to 2 in 10. And if both parents are left-handed, the child has a 4 in 10 chance of being left-handed.

If a classic dominant-recessive model were in play, then left-handed people would only have left handed kids. But they don't even have left-handed kids most of the time!

What's going on here is that genes aren't the whole story. Good evidence that something besides genes are involved comes from looking at identical twins.

Remember, identical twins start out with exactly the same DNA. If something is completely determined by genetics, then if one identical twin has it, the other in the pair should as well.

When one twin is left handed, the other twin is left handed only 76% of the time. Clearly genes aren't enough. So, what else is involved?

Most likely, the environment plays a role. There are probably one or more genes that make you more likely to become left-handed. You then need some sort of environmental trigger for it to happen.

This is what happens with, for example, juvenile diabetes. Scientists believe that not everyone who has the genes necessary for diabetes get it. A current theory is that you need certain genes and a viral infection to get the body to attack its own pancreas and give itself diabetes.

But is there any evidence for a trigger with handedness? I couldn't find much research in people, but some interesting studies have been done with chimpanzees.

In chimpanzees, being right handed is about twice as common as being left handed (as opposed to around 9 times as likely in humans). When scientists studied chimpanzees, they found that the chances for having a left-handed baby increased significantly based on birth order. The first and sixth or later babies had a much higher chance of being left-handed than did the middle ones.

What do the first and later pregnancies have in common? They tend to have hormonal variations that don't happen with the middle pregnancies. And this might be the environmental trigger we're looking for.

There is some evidence in humans that birth order may play a role as well. Nothing as clear-cut as we see with chimps but we do know that older mothers tend to have more left handed children. Also, at least with boys, some studies have shown that later kids are more likely to be left-handed.

So, handedness is most likely due to a combination of genes and environment. Some people have a greater chance of being left-handed but something needs to trigger it.

Remember, the identical twin story argued against genes being the whole story. It also argues that birth order isn't everything either. If it were, then if one identical twin was a lefty, the other would always be one as well since they are born at the same time. Birth order might be one trigger but there must be others as well.

By Dr. Barry Starr, Stanford University