Abstract
At present, we do not understand how human handedness arose. Although much attention has been paid to several genetic theories, some studies involving twins have indicated that the influence of genetic and environmental factors on handedness may be minimal or nonexistent. The incidence of left-handedness amongst males is higher than with females, according to a large body of research. People who are ambidextrous are more likely to start to write with their right hand, even when they are not forced to do this. The left hemisphere of the brain is also lateralized when compared to the right hemisphere and vice versa. Our research examined handedness in dizygotic and identical monozygotic twins in Koya Town/ Erbil governorate. In 2014, we mailed a questionnaire to 100 twins aged between 18 and 50 to survey their handedness. The survey determined whether each subject was a monozygotic or dizygotic twin, and then questioned them about their handedness. We examined whether both twins had the same handedness or different handedness, and whether individual twins have a higher prevalence of left-handedness or right-handedness. The results revealed that in 62% of cases, both monozygotic twins in a pair are right-handed. On the other hand, both dizygotic twins are right-handed only 54% of the time. Furthermore, dizygotic twins are more likely to have one left-handed and one right-handed twin when compared to monozygotic twins. We also found that the prevalence of right handedness in individual monozygotic twins regardless of the handedness of the other twin was 77%, whereas the prevalence of left-handedness was only 21%. The remaining 2% of monozygotic subjects were ambidextrous. These results indicate that unique genetic and environmental factors may account for the majority of variance in that is observed in handedness, although the relationship between the brain, handedness, environmental effects and genetic effects is a complex one. This is a view that is corroborated by a number of recent neuroscientific and genetic studies that are examined in this article.
Highlights
Each hemisphere of the brain specializes in processing sensory information that is not processed in the other hemisphere (Forrester et al, 2014)
The survey determined whether each subject was a monozygotic or dizygotic twin, and questioned them about their handedness. We examined whether both twins had the same handedness or different handedness, and whether individual twins have a higher prevalence of left-handedness or right-handedness
We found that the prevalence of right handedness in individual monozygotic twins regardless of the handedness of the other twin was 77%, whereas the prevalence of left-handedness was only 21%
Summary
Each hemisphere of the brain specializes in processing sensory information that is not processed in the other hemisphere (Forrester et al, 2014). This functional split, which is found in both invertebrates and vertebrates, is posited to provide an evolutionary advantage by increasing neural processing efficiency (Gu et al, 2013). The right hemisphere of the brain controls the left side of the body and vice versa (Knecht et al, 2000). Specializations in one hemisphere can give rise to physical effects on the opposite side of the body. Handedness is the most well-known example of this shared hemispheric and anatomical lateral bias in humans (Joseph and Hellige, 1993)
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