The Genetics Of Handedness

Given the evidence that handedness patterns in humans extend far back into evolutionary history, it is not surprising that the vast majority of theories of handedness have included the suggestion of a genetic factor. There have been a number of studies that have examined handedness in families in order to verify this hypothesis. Unfortunately, the empirical evidence on handedness does not strongly support genetic theories.

There is a fairly consistent finding that left-handedness is more likely in children of left-handed mothers. However, when we do the fine-grain analyses needed to determine that a characteristic is inherited, the numbers relating the father's handedness to that of the offspring, the relationship between brothers and sisters, or predictions from the grandparent's handedness just do not seem to fit the patterns required by most genetic models. Some theorists are still trying to work out more sophisticated mathematical descriptions that might show that handedness is inherited; however, there is still a strong element of doubt in the data.

Perhaps the most compelling evidence against a simple, strong genetic determinant for handedness comes from the results of studies that compared the handedness of monozygotic or identical twins with those of dizygotic or fraternal twins. Twin studies are usually considered as providing the clearest indication of the presence or absence of inherited components for most behavioral traits. Monozygotic twins have an identical set of genes since they come from a single egg, fertilized by a single sperm, that later splits into two individuals in the early stages of cell division. Dizygo-tic twins come from two egg cells fertilized by different sperm; hence, they share only the level of genetic similarity that we would find between any pair of brothers or sisters. This means that, at a minimum, if there is a genetic component in handedness, one would at least expect that pairs of monozygotic twins would be more likely to have the same handedness than dizygotic twins. Figure 2 summarizes the results of 16 studies that compared the handedness of twins. Notice that there is no difference between monozygotic twins and dizygotic twins with regard to the likelihood that they will share the same handedness. Even more striking is that the likelihood that any form of twins will have the same handedness is no greater than if we randomly chose pairs of unrelated individuals and determined whether or not they had the same han-dedness.

This is not to say that genetics has no part in the determination of handedness. Certainly, genetic factors determine many human characteristics, such as the number of eyes, ears, and limbs that characterize our species. We might refer to these as genetically fixed characteristics, which are expected in all members of the species. We might contrast these characteristics to genetically variable characteristics, such as eye color, that do vary in the population, depending on the specific genes transmitted to the offspring by its parents. There is evidence that suggests that the

Figure 2 The number of same-handed pairs of twins does not differ among identical twins (who share identical gene sets) and fraternal twins (who are as genetically similar as any pair of siblings), and the twins are no more similar in their handedness than are unrelated pairs of individuals.

strength or consistency of handedness is inherited from parents and, hence, is genetically variable. Children of parents with inconsistent or mixed handedness (tending toward ambidexterity) are more likely to have children with mixed handedness than are parents with consistent handedness. However, whether one is left- or right-sided seems to be a genetically fixed characteristic of species, with evolution programming humans to be right-sided. If right-handedness was meant to be a characteristic of species, then why are some people left-handed?

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