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Key points
- Left-handedness is influenced by environmental factors, but the molecular processes in this association are unclear.
- A new study investigated the role of epigenetics for left-handedness.
- Results show a very small effect of DNA methylation in blood samples on left-handedness.
About 10.6 percent of people are left-handed. If you prefer your left hand for activities like writing and drawing, you might have wondered what made you that way. It was likely a combination of genes and environment; a large-scale study on twins and their families has shown that about 25 percent of the individual variance in handedness can be explained by genes, while 75 percent is determined by environmental influences (Medland et al., 2009).
Influences that have been shown to affect handedness include cultural effects influenced by the year and location of birth—for example, older people are less likely to be left-handed since there used to be pressure against left-handedness in school. Moreover, the probability of being left-handed is affected by birthweight, being one of multiple births, the season of birth, breastfeeding, and sex (De Kovel et al., 2019).
An Open Question About Left-Handedness
One of the most important open questions in left-handedness research is how exactly these factors influence left-handedness. A new study led by Veronika V. Odintsova from the University of Amsterdam focused on a potential molecular mechanism that could affect left-handedness (Odintsova et al., 2022).
To do this, she conducted a so-called epigenome-wide association study of left-handedness Epigenetics is a term used to describe several different molecular processes that modify how specific genes within the genome are expressed—how the information in a gene is used to synthesize a protein. Epigenetics is a major process in how non-genetic factors influence the structure and function of the brain or other parts of the body. Since left-handedness is caused by the brain, it could be hypothesized that factors like birth weight affect epigenetic processes that influence the expression of genes that influence brain structure.
Odintsova analyzed blood samples of 3,914 adult volunteers (about 10.8 percent left-handers and 89.2 percent right-handers) and investigated DNA methylation in these samples. DNA methylation is one of the most important epigenetic processes in humans. In short, methylation is a process in which a small molecule, a so-called methyl group, is added to a specific location in the DNA. This addition of the methyl group can “switch off” or reduce the activity of the gene it is attached to without changing the gene itself. Thus, it works a bit like a light switch that can be used to switch off a light bulb.
What Did the Researchers Find Out?
Odintsova and her co-workers compared methylation at 409,562 locations in the genome between left-handers and right-handers. Surprisingly, there were absolutely no statistically significant differences between left-handers and right-handers in any of these locations.
The scientists then conducted a second analysis in which they analyzed larger regions of the genome instead of single locations. In this analysis, one region on chromosome 2 and one region on chromosome 20 reached significance, with left-handers showing less methylation in these regions than right-handers, but the differences were tiny (0.1 percent and 0.4 percent).
The genes within one of these regions were associated with brain development and nervous system structure maturation and maintenance, among other things. The genes within the other regions, interestingly, were associated with neurological and psychiatric traits, including schizophrenia. This could be relevant to explaining the finding that left-handedness and mixed-handedness are observed more often in people with schizophrenia than in the general population.
While these findings were interesting, the overall effect of DNA methylation in blood samples on left-handedness was very small. The scientists concluded that it captures little variance in handedness.
One major issue the scientists remarked on was the fact that they used blood samples to investigate DNA methylation. However, left-handedness originates in the brain, not the blood. Since different tissues are known to show different methylation patterns, the authors remark that future studies should aim at directly investigating brain tissue in epigenetic research on left-handedness.
Facebook image: Lyubov Levitskaya/Shutterstock
References
de Kovel CGF, Carrión-Castillo A, Francks C. (2019). A large-scale population study of early life factors influencing left-handedness. Sci Rep, 9, 584.
Medland SE, Duffy DL, Wright MJ, Geffen GM, Hay DA, Levy F, van-Beijsterveldt CE, Willemsen G, Townsend GC, White V, Hewitt AW, Mackey DA, Bailey JM, Slutske WS, Nyholt DR, Treloar SA, Martin NG, Boomsma DI. Genetic influences on handedness: data from 25,732 Australian and Dutch twin families. Neuropsychologia 2009, 47, 330-337.
Odintsova VV, Suderman M, Hagenbeek FA, Caramaschi D, Hottenga JJ, Pool R; BIOS Consortium, Dolan CV, Ligthart L, van Beijsterveldt CEM, Willemsen G, de Geus EJC, Beck JJ, Ehli EA, Cuellar-Partida G, Evans DM, Medland SE, Relton CL, Boomsma DI, van Dongen J. (2022). DNA methylation in peripheral tissues and left-handedness. Sci Rep, 12, 5606.
