Is Left-Handedness Inherited From The Parents?
One of the many theories being researched on suggests that handedness is a Mendelian trait. What is Mendelian trait?
Taken from Wikipedia.com, “A Mendelian trait is one that is controlled by a single locus and shows a simple Mendelian inheritance pattern”.
To put it into simpler words, a simple Mendelian trait is one where there is a dominant gene and a recessive gene. One copy of the gene comes from dad and the other from mum. Take handedness as an example: If right-handed (Rg) is a dominant gene, the baby will be right-handed if both the copies are Rg and either one of the copy is Rg. Only if both the copies are left-handed (Lg) will he or she be left-handed.
Fig 1. If handedness is a Mendelian trait with Left-Handed being the recessive copy
If the classic dominant-recessive model as above is true, it means that if both parents are left-handed, then definitely their child will be left-handed too. That is not always the case. We can be sure that something else is in play here that determines handedness by studying identical twins. Twin starts off with the same set of DNAs and theoretically, if one of them is a left hander, the other should also be one. Yet statistically, the second one only has ~76% chance of being a left hander.
Adoption studies suggest that handedness is under genetic control as the results indicate the handedness of adopted children is more likely to follow that of their birth parents than their adopted parents.
That being said, if you and your spouse are both left-handed, chances are your child will be one too, and that is something to be proud of. Beside genetics, environmental pressure is now considered to play a part as well, for parents may purposefully or incidentally teach their children the handedness they themselves and not the children preferred. One or more genes might be responsible for left handedness, but a trigger has to occur from the external to activate this trait.
Other researchers have explored the idea that left-handedness is produced by a brain abnormality, such as a neural defect, perhaps caused by a diminished blood supply to the left hemisphere during fetal growth. Early brain damage in the left hemisphere can result in a shift of language and dominant limb function to the right hemisphere. There does seem to be a correlation between premature birth, prolonged labor, breech births, and the tendency toward left-handedness.
Another theory, as yet unsubstantiated, suggests that the male hormone testosterone might be responsible for handedness. This hormone slows left hemisphere development in the male fetus and could account for left-handedness, but it does not account for female.
This subject becomes even more intriguing when we consider the fact that the right-sided tendency is species-specific to humans. While some animals are right or left-pawed in relatively equal numbers, only humans show this large percentage with right-hand preference.
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