Scientific research into the genetics of handedness has been conducted for over 100 years. Genes are sequences of DNA in specific locations on one of the 23 chromosome pairs of the human genome. A gene can have two alleles, one from each parent, that determine a genetic trait.
Studies conducted in the early 20th century assumed that left-handedness, because it is relatively rare, is a genetically recessive trait. The presence of the dominant allele for right-handedness suppresses the expression of the recessive trait of left-handedness. A person must have two alleles for the recessive left-handedness trait for it to appear. Geneticists test their theories about the genetic transmission of handedness by comparing the actual percentages of right- and left-handers found in human populations with the percentages predicted by their genetic models. Unfortunately, a simple dominant-recessive genetic model does not provide a good fit for the observed population percentages of right- versus left-handers.
The search for a specific handedness gene did not stop because the models of the early 20th century were flawed. Later theories argue that the two alleles for the handedness gene are called, R, for right-handedness, and C, for chance. If a person does not inherit the R gene then handedness is determined by chance. In other words, 50% of people with two C alleles are right-handed while the remaining 50% are left-handed. Models based on this approach are more successful in predicting the population percentages of right- and left-handers. They can also explain the existence of right- and left-handers in the same family. If the C allele is part of a family’s genome, the opportunity exists for chance factors to intervene to determine handedness side.
Other genetic theorists claim that a specific gene for determining handedness side does not exist. Rather there is a gene that controls the direction of body asymmetry. Left hemisphere control of both speech and right-hand movements are the dominant human asymmetries. Most people inherit a gene that shifts asymmetry toward the right side. If a person does not inherit this right shift tendency, handedness and hemispheric control of speech are determined by chance. This theory can explain the overwhelming majority of right-handed, left-brained humans and it is consistent with the evidence that specific genes control the direction of asymmetry in humans. For example, there is a genetically-determined condition called situs inversus where the internal organs of the human body are reversed from their normal positions…the heart is on the right rather than the left side and the liver and gall bladder are on the left rather than the right. Situs inversus is one outcome of a gene controlling the asymmetrical locations of internal organs. Geneticist argue that there are possibly other genes that determine asymmetrical handedness preference.
The 21st century has seen the mapping of the human genome. Genome-wide scans of individuals can inspect gene locations on specific chromosomes. Geneticists interested in handedness use this technique to compare the genomes of right- and left-handers. The current thinking is that as many as 40 genes related to the control of nervous system development and body asymmetry may contribute to the determination of human handedness. If the genomes of right- versus left-handers differ at these gene sites then scientists may be able to identify specific genes that play a role in the determination of handedness.
Why has genetic research played an important role in the science surrounding human handedness? Perhaps because archaeological evidence indicates that right- and left-handedness have been present in human populations for millennia. Not only have our humanoid ancestors shown handedness but the trait has occurred in the same population proportions, 85-90% right and 10-15% left, that exist today. Geneticists call this situation a balanced polymorphism where two forms of handedness, right and left, have remained constant in their respective percentages for many centuries. This historic stability leads scientists to investigate genetic endowment as one of the factors contributing to the development of the handedness trait.