“Handedness” describes which hand—left or right—an individual typically uses when performing one-handed tasks. Right-handed people seem to be predisposed to use their right hands when writing, playing ping-pong, punching, throwing objects, etc. Approximately 87% of the adult population is right-handed.
Scientists have long searched for the underlying cause behind handedness, speculating that genetics, prenatal trauma, and environmental factors may have a hand in the trait. In 2007 a study was finally published which linked handedness to a gene, “the first potential genetic influence on human handedness to be identified” (Franck et al., 2007).
The study was published online in the journal Molecular Psychiatry on 31 July 2007. The paper (which can be found online here) describes the association between handedness and a haplotype upstream of the gene LRRTM1. The abstract of the article gives the context for the discovery, describing the gene’s association with several neurodevelopmental disorders, as well as the trait’s association with the asymmetry of brain function.
On the same day, the BBC News published an article announcing the discovery of a “gene for left-handedness” (BBC News, 2007). The BBC article cites the gene LRRTM1 as the gene that decides handedness, a slight misreading of the original study that associated handedness with the haplotype, or close set of genes inherited together, upstream of LRRTM1, not LRRTM1 itself. However, the popular press article does a good job of extracting the idea that the gene seems to influence which side of the brain controls which asymmetrical functions.
The BBC article also projects more certainty on the discovery than is given in the scientific article. The headline alone, “Gene for left-handedness is found” (BBC News, 2007, is enough to mislead, suggesting both that the gene is decisively the cause of left-handedness, and that only one gene determines the trait. Neither fact is proven. Furthermore, the BBC article says nothing about the test group of the scientific study, which was limited to a group of dyslexic siblings and a group of schizophrenic or schizoaffective individuals; the abstract explicitly states that the scientists were unable to find the same effects in a study of “twin-based sibships”, and qualifies its claims with words like “potential” and “putative” where the BBC article does not (Franck et al., 2007).
The gene LRRTM1 is a leucine-rich repeat transmembrane neuronal protein; it contains 10 sections of leucine-rich repeats. There is a wealth of information about the gene on NCBI's Entrez Gene website. Homologous genes are found in several mammals, from chimpanzees to zebrafish. LRRTM1 is an imprinted gene, meaning that the parent of origin determines the gene expression. In this case, the correlation of handedness with the gene is passed down paternally (NCBI Entrez Gene, 2008).
The protein is a transmembrane protein, and while no PDB file exists for the LRRTM1 protein, we can predict its structure based on a similar protein product:
A Jmol image of the similar Nogo-66 receptor protein with leucine-rich repeats (from PDB):
It is important to remember that the gene/protein LRRTM1 does not cause the phenotype of handedness on its own. Many factors interact to determine brain hemisphere function and handedness. Chimpanzees have the gene LRRTM1 as well and—what’s more—it is 100% identical to the human version. Chimps, however, show no signs of handedness; their lack thereof suggests that environmental factors and other genes are undoubtedly key factors in the expression of handedness (Gerritsen, 2008).
Handedness most likely evolved as a result of differentiated brain function between the hemispheres. In most individuals, the left hemisphere controls speech and language, while the right side controls emotions. In left-handers' brains, the two sides appear to switch responsibilities, thereby also leading to a switch in the dominant hand. The link between handedness and separation of brain function seems clear. Some scientists speculate that this separation of brain function arose as humans began to communicate with speech, increasing selection pressure for speech-capable brains, which seem to rely on the separation of hemisphere function, and along with brain structure came handedness. An excellent summary is given here:
Protein Spotlight: The Hands to Say It
BBC News | Health. 2007 July 31. "Gene for left-handedness is found". <http://news.bbc.co.uk/2/hi/health/6923577.stm>. Accessed September 17, 2008.
Francks C, Maegawa S, Lauren J et al. 2007 July 31. LRRTM1 on chromosome 2p12 is a maternally suppressed gene that is associated paternally with handedness and schizophrenia [abstract]. In Molecular Psychiatry (2007) 12, 1129–1139. <http://www.nature.com/mp/journal/v12/n12/abs/4002053a.html>. Accessed September 17, 2008.
Gerritsen, Vivienne Baillie. 2008 February. The Hands to Say It. Protein Spotlight. <http://www.expasy.org/spotlight/back_issues/sptlt091.shtml>. Accessed September 17, 2008.
Harrison R.M, Nystrom P. 2008 Jan 22. Handedness in captive bonobos (Pan paniscus) [abstract]. In Folia Primatolo (Basel). <http://www.ncbi.nlm.nih.gov/pubmed/18212503?dopt=Abstract>. Accessed September 17, 2008.
NCBI Entrez Gene. 2008 July 22. LRRTM1 leucine rich repeat transmembrane neuronal 1 [ Homo sapiens ]. <http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=347730>. Accessed September 17, 2008.
Protein Data Bank. 2008 Sep 16. 1OZN. <http://www.pdb.org/pdb/cgi/explore.cgi?pdbId=1OZN>. Accessed September 17, 2008.
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