Neuroscience for Kids

Genetic Control of Brain Symmetry

By Melissa Lee Phillips
Neuroscience for Kids Consultant
May 7, 2002

Broca's Area

Wernicke's Area

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Slice of Life.
Scientists have found that the brains of left-handed people are shaped differently than those of right-handed people. Right-handers usually have asymmetric brains; their left cerebral hemispheres tend to be larger than their right hemispheres. In particular, Broca's and Wernicke's areas, two regions involved with language, are likely to be much larger in the left hemisphere than in the right. In contrast to right-handers, left-handers generally have more symmetrical brains, with similarly sized language centers in each hemisphere.
One possible explanation for this observation is that most right-handers have a "right-shift" set of genes. When these genes are active, the person will be right-handed and most likely will have a larger left cerebral hemisphere. If a person does not have active forms of these "right-shift" genes, there will be no genetic control over handedness or brain symmetry. These people can be either left- or right-handed, and their brains will be more symmetrical. A study by Daniel Geschwind and his colleagues at the University of California in Los Angeles adds support to the theory of genetic control over brain symmetry.
Geschwind and his team measured the volume of the left and right cerebral cortex in identical male twins and compared these volumes to handedness. Consistent with previous research, the left-handers had more symmetrical brains than the right-handers. Additionally, they compared the hemispheric volumes in each set of twins and found that right-handed twin sets were more likely to have similarly shaped brains than were twin sets in which at least one man was left-handed. These results support the "right-shift" genetic theory. It seems that there is genetic control over the asymmetry present in the brains of most right-handers. The loss of asymmetry in left-handed people may reflect a lack of this genetic control.

Reference and more information:

Geschwind, D.H., Miller, B.L., DeCarli, C., and Carmelli, D., Heritability of lobar brain volumes in twins supports genetic models of cerebral laterality and handedness, Proc. Natl. Acad. Sci. USA, 99: 3176-3181, 2002.
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