Is Left-Handedness Hereditary?

The other day while I was at work, I had an interesting conversation with one of my co-workers. I passed by him as he was writing out a check to pay for a delivery, and he remarked that the check looked as if a kindergartener had written it. He went on to explain that the business check register is designed for use by right-handed people and since he is left-handed, he has trouble writing neatly on the checks. This random moment from my daily life got me thinking about left-handedness.

Handedness is thought to be hereditary, although the exact mechanism by which the trait is inherited is not known. If both parents of a child are left-handed, there is a slightly less than thirty percent chance that the child will be left-handed. In some families, however, there are quite a few left-handed people. For example, the British Royal Family boasts an unusually high percentage of left-handers.

One of the curious things about handedness is that there is not much that can be conclusively stated about whether it is definitely hereditary. If it is hereditary, then we seem to know more about how it is not inherited than how it is inherited. Handedness is not a Mendelian trait because it does not involve a dominant and a recessive gene. It appears to be one of those hereditary situations where possessing a certain gene makes it more likely that you will exhibit the trait, but does not guarantee that you will. Studies that have been done on handedness in twins suggest that although genetics may play a role in handedness, there seems to be something more going on. If one twin is left-handed, the other twin is left-handed only seventy-six percent of the time. This “something more” is thought to be an environmental trigger that tips the balance one way or another. Of course, exactly what that environmental trigger is remains a mystery.

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