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February 21, 2024

Scientists discover genetic basis for handedness

By MELISSA STALEY | September 27, 2013

The distribution of right handed and left handed people in the world seem to be random. Different family members can write with either hand, and there are a few of those who can switch between both hands. In most cases, it doesn’t seem to be a trained practice and it just comes naturally for each individual. However, recently, genes that are linked to the orientation of internal organs during embryogenesis were found to play a role in the development of handedness.

According to a study published in Plos Genetics, these genes are fundamental to the development of left-right asymmetry in embryos and may also help establish asymmetry in the brain. Researchers believe that cerebral variances in particular may play an important role in the development of handedness.

Around the world, 85 to 90 percent of individuals prefer use of their right hand as opposed to their left. Strangely, this same degree of preference is not seen in any of the great apes and appears to be an exclusively human trait. The reason behind this bias remains largely unclear. Studies show that this may have to do with the development of asymmetries of the motor cortex and language centers in the brain.

Because handedness is cross-linked, preference of left hemisphere dominance in the motor cortex would result in right handed individuals. Left hemisphere language dominance is also more common even among left handed individuals. A strong link between language and the development of handedness may influence the development of right hand dominance.

Others believe that the development of handedness may have allowed humans to perform tasks more effectively but are unsure why right handedness would be evolutionarily beneficial compared to left handedness.

Eager to examine the genetic basis between right versus left hand dominance, researchers led by Dr. Silvia Paracchini at the University of St Andrews, conducted a genome wide association study. They hoped to shed light on the genes linked to handedness and the development of handedness in humans.

In the study, researchers had test subjects move a series of pegs with both their right and left hands. Analyzing the speed at which each task was done allowed the subjects’ dominant hand to be determined. When studying the genomes of these subjects, variants in the gene PSCK6 and several others were identified that correlated well with the handedness of the individuals.

PSCK6 is a gene responsible for the regulation of NODAL, an enzyme important in the development of left-right asymmetry in most bilateral organisms. Researchers hypothesize that NODAL and other proteins involved in its pathway are important for determining handedness in individuals.

Disruption of PSCK6 in mice has been found to lead to a variety of organizational disorders including situs inversus, the inversion of organs to mirror their normal orientation and left-hand isomerism, the replication of the left side on both sides of the body.

When the gene is deactivated completely, heterotaxia or the random placement of internal organs, results. Researchers acknowledged that PSCK6 is one of possibly hundreds of genes that lead to handedness in humans. They explained that handedness is a polygenic trait and one that may also have a multifactorial basis — one in which genetics and environment both play a role.

Even so, we are one step closer to understanding the genetics behind handedness in humans. Only further research will truly reveal the complex mechanisms that influence this trait.

 


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