The Left-Hander in a Right-Handed World
In this blog, we’ve discussed many life characteristics influenced by genetics.
These are things you encounter when writing a genetics blog.
Would you have expected one of them to be being genetically left-handed?
A left-hander, regardless of their species, is an individual who has a natural or acquired tendency to preferentially use the left side of their body, primarily their limbs.
Left-handers can be homogeneous if this left-sided tendency is reflected in almost all their organs, or partial, in which case they exhibit crossed laterality: some parts are predisposed to the left, and others to the right.
A partially left-handed person might favor their left side while their dominant leg is on the right.
There are exceptional cases. Tennis player Rafael Nadal is right-handed but plays tennis with his left hand because it gives him an advantage against opponents who are more accustomed to facing right-handed rivals. This makes him a right-hander who becomes left-handed when playing tennis.
There is one final group: ambidextrous people. They fall into two categories: those who can use both the left and right hands interchangeably, and those who choose which hand to use depending on the task. They make up about 1% of the world’s population.
Ambidextrous individuals are often left-handed from birth but adapt to a predominantly right-handed society.
Only about 8% to 13% of the world’s population is left-handed.
The actual percentage may be higher, as many people have been forced to become right-handed due to social, material, and cultural pressures.
We won’t simplify it by saying that being left-handed is entirely genetic, but DNA does try to play a role wherever it can.
The Biology of Left-Handedness
Being left-handed is a complex trait. It’s considered to involve both genetic and environmental factors and is something almost unique to our species.
Evolution has attempted to create symmetry in living beings but cannot help favoring laterality in some cases. Most of that laterality leans toward the right.
A simple example can be seen in mollusks. Many species have a shell that must spiral in one direction, and it’s usually to the right (in 90% to 99% of gastropod species).
Scientific studies have shown that this is due to genes that code for formins, actin-binding proteins involved in the polymerization of actin filaments. The inherited gene led to left-handed individuals when both parents were also left-handed.
What’s interesting is that modifying this genomic region during embryonic development could cause the offspring of right-handed parents to develop a left-handed shell.
In 2020, the gene responsible for this shell laterality in the snail species Lymnaea stagnalis was confirmed. The gene, Lsdia1, encodes a formin.
Formins are present in all eukaryotic cells and are highly conserved evolutionarily. Their coding sequence is largely shared among all eukaryotic organisms.
Thanks to this, researchers demonstrated through experiments that formins also controlled right-left asymmetry in the vertebrate model Xenopus laevis, a species of frog.
The existence of species with a predominant side has more significant implications than it might initially seem. Its effect extends to other animals.
If most snails have shells that spiral to the right, wouldn’t it be logical for certain predators, such as crabs, to evolve to be right-handed, making it easier to extract them from their shells?
Another interesting concept is that laterality emerges at an embryonic stage.
Some experts argue that body asymmetry might have already arisen in unicellular organisms. Although calling a cell a “body” might be a bit of a stretch.
Left-Right Asymmetry in Humans
In biology, there is a concept called left-right asymmetry. This is the process that determines, during an individual’s development, the left and right sides of their body. It holds immense importance. In humans, for instance, this asymmetry ensures that the heart develops on the left side.
Many genes are involved in this process. The absence of the PITX2 gene causes several organs to appear on the opposite side and is one of the factors responsible for heart asymmetry. The genes Lefty1 and Lefty2 are essential for determining the left-right axis in the embryo. Mutations in the NODAL gene result in random positioning of visceral organs.
There is a condition called situs inversus that alters the positioning of organs within the body. People with this condition, about one in 10,000, have their organs on the opposite side of the body, as if in a mirror image. This condition is congenital and genetic.
The human brain also exhibits lateral asymmetry. The two hemispheres of the brain have structural and functional differences.
At some point, it seems that the central nervous system decided it was better to specialize certain tasks rather than divide them equally.
For example, language is predominantly associated with the left hemisphere, which houses Broca’s and Wernicke’s areas.
An interesting detail is that hemispheric control of the body is crossed: the right hemisphere controls the left side of the body and vice versa. This is why a right-handed person is considered to have a dominant left hemisphere.
These asymmetries are not absolute but gradual. One hemisphere may be more involved in a particular action, but not the sole one responsible.
This is why some have tried to justify the predominance of right-handed people over left-handed ones based on language control. Since speech requires highly refined control, the hemisphere responsible for speech might have specialized in all tasks requiring precision.
The problem is that in most left-handed individuals, the left hemisphere is still responsible for language. Simple solutions don’t work here.
At least the genetic basis for left-handedness is undeniable. 26% of children with left-handed parents are also left-handed. There is a genetic inheritance for left-handedness.
It is known that the genetic loci involved contain genes that encode proteins that determine left-right asymmetry and brain development.
Twin studies have estimated that genetic factors account for 25% of the predisposition to being left- or right-handed, while 75% is attributed to environmental factors.
Ultimately, the best way to confirm if someone is left- or right-handed is to ask them to write something. However, the tellmeGen genetic analysis will give you a predisposition so you can see if environmental factors are hiding your genes.
