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Top Five Questions Left-Handers Ask, #2: Is Laterality Genetic?
Recently I gave a talk titled The Mystery of Left-Handedness at Lifeworks an organization in Erie, PA that promotes life-long learning, health and wellness (see lifeworkserie.org). My talk was organized around questions about left-handedness I have been asked repeatedly.
My five questions below represent issues that arise again and again when I talk to left-handers or when I am interviewed by the media about left-handedness. I will answer each of these five questions in my five-part blog series of the next several weeks.
- I write with my left hand but do other things with my right hand. Is that normal?
- Everyone in my family is right-handed but me. Why? I thought handedness is genetic.
- Are left-handers smarter and more creative than right-handers?
- If I try to switch writing from my left to my right hand, will I mess up my brain?
- Are left-handers better at sports than right-handers?
Response to Question Number 2:
It is not uncommon for families to contain both right- and left-handers. In the later years of the 20th century, one popular genetic theory argued for a handedness gene that had 2 alleles. Allele is the genetic term for the alternative forms of a gene at the same genetic location on a chromosome. The two alleles affecting handedness are D for dextral or rightand C for chance. Since a person receives half of his/her genetic material from the father and half from the mother, several different genetic combinations (genotypes) that affect handedness can occur. People with DD genotypes are all right-handed, about 75% of people with DC genotypes are right-handed while the handedness of the people with the CCgenotype is determined by chance, 50% are right-handed and 50% are left-handed. This theory can account for the predominance of right-handedness in human populations since 2 of the genotypes result in all or mostly all right-handers. It can also account for the presence of right- and left-handers in the same family. If the C allele is part of a family’s genotype, chance plays a major role in handedness determination and left-handedness is one of the results of this chance intervention.
The 23 Human Chromosome Pairs
Genetic research on handedness has taken a different approach in the 21st century since the completion of the mapping of the human genome. The figure above shows the 23 chromosome pairs that compose the human genome. The bands of light and dark on each of the chromosomes indicate a genetic site. Individual genetic sites on different chromosomes have been identified as possible candidate genes contributing to handedness. These genes may not determine handedness directly but are genes related to brain development or to the formation of bodily asymmetries such as the left-right positions of internal bodily organs.
In 2014, I visited Chris McManus in London, England. Chris, who is a faculty member at University College London, is one of the world’s leading experts on the genetics of handedness and one of the editors of the leading journal in the field, Laterality. He described one study in progress that takes advantage of human genomic databases and the sophisticated programs now being used to compare genomes. The idea is to compare the genomes of family members from families with large numbers of left-handers to the genomes of families without left-handers. Since candidate genetic sites have been identified, researchers can compare these sites for similarities or differences between right- and left-handers within and between families. Chris estimates that as many as 30 to 40 genetic sites may contribute to the development of handedness side.
Research into the genetics of handedness has a long history extending back to the 19th century. The mapping of the human genome has shifted thinking about the genetics of handedness, opening possibilities for exciting new findings in the future. Genetic researchers have been plagued with the conundrum of trying to account for the preponderance of right-handedness among humans while simultaneously explaining the persistent presence of a minority of left-handers. Genomic comparisons may unravel the complex mystery of what are and where are the genes that contribute to handedness formation.
In summary, it is not uncommon for families to contain both right- and left-handers.
Stay tuned for the next articles appearing throughout the end of November for answers to the next questions.
Author Interview: Could forcing yourself to write with BOTH hands make you smarter?
ABOUT THE AUTHOR:
Clare Porac received her Bachelor of Arts degree in sociology from Duquesne University and her MA and PhD degrees in psychology from the New School for Social Research, and is presently Professor of Psychology at Penn State Erie. She has authored or coauthored 63 research articles and has presented 66 conference papers on her human laterality research; she has an additional 55 publications and 50 conference papers on other topics. You can read her full biography here.
Professor Porac’s new book, Laterality: Exploring the Enigma of Left-Handedness, is a comprehensive overview of scientific research on laterality that not only tells us what is true, but also debunks commonly held misperceptions. Each chapter is based on a question or questions covering diverse topics such as genetic and biological origins of handedness, familial and hormonal influences on handedness, and the effects of a majority right-handed world on the behaviors of left-handers.
If you would like to purchase a copy of Laterality: Exploring the Enigma of Left-Handedness at up to 30% off the list price and free global shipping, visit the Elsevier Store. Apply discount code STC215 at checkout.
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