10% of the population prefers to write with their left hand, revealing that the branching of the human brain remains a mystery.
A newly published research study has shed light on the interplay between genetics and chance that underlies this seemingly innocuous characteristic.
Handedness is shaped from the early weeks of pregnancy. Some rare genes are found nearly three times more often in left-handed individuals, providing us with insight into the mechanisms behind this branching.
Only 10% of the population is left-handed. Clyde Francks, Director at the Max Planck Institute (Netherlands) explains, “We know that the two hemispheres of the brain begin to develop differently in human embryos, but the mechanisms behind this are still unknown. It is likely that right-handedness is the default outcome of early brain development encoded by our genes. But what happens with left-handed individuals despite these tendencies?”
Only 10% of the world’s population is left-handed. (Illustration: Science et Avenir).
What Determines the Probability of Being Left-Handed?
The answer may partly lie in the tools that cells use to classify and route proteins.
Specifically, some DNA sequences regulate strands of proteins known as microtubules, a type of “scaffolding” for movement, which have been identified as one of the most common genetic variants in left-handed individuals.
Clyde Francks states: “These types of genetic variants often have a minor impact on human traits, such as influencing the activity of a specific gene in a particular tissue.”
For this reason, in the new study, scientists focused on genetic variants that directly affect the sequences of proteins—not the regulatory sequences.
The researcher adds that while rarer, these types of genetic variants can have a much greater impact on human traits—in a small number of individuals who carry them.
By examining the genomes of nearly 350,000 people from the UK Biobank database, researchers identified a mutation, although rare, that is 2.7 times more common in left-handed individuals.
This mutation affects the TUBB4B gene, which encodes for microtubules. Clyde Francks notes: “Thus, this is a nice convergence of evidence—supporting the important role of microtubules in human branching.
Rare genetic variants in a small number of individuals may highlight genes that provide clues about the mechanisms behind the development of brain asymmetry in everyone.”
Is Handedness Random?
Researchers point out that there is a significant random factor in human branching.
“We believe that in most cases, the preference for left-handedness is simply due to random variations during embryonic brain development, without specific genetic or environmental influences.
For example, random fluctuations in the concentration of certain molecules during critical stages of brain formation”, Clyde explains.
Therefore, rather than directly determining who is left- or right-handed, DNA creates a “strong tendency” favoring one side or the other, largely biased towards the right naturally.
For instance, microtubules present in cilia—a type of small appendage on the cell surface—may be the source of asymmetric movements and fluid flow, contributing to the left-right axis in the embryo.
Rare variants of the TUBB4B gene are believed to disrupt this process in a way that is less biased towards the right than the left, making those in this category more likely to be left-handed.
The researcher argues that with this element of randomness, genetically identical twins can also have the same dominant hand.
