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Scientists and laymen alike have often wondered why nearly one in ten children prefer to use their left hands over their right ones. Being left-handed does not significantly increase one's ability to function in society, in some cases left-handedness can be incredibly inconvenient. If handedness were entirely random, you would expect 50% of the population to prefer their left hand. Why does it persist in society at such a low frequency? Is it taught or is it genetic? A single gene for hand preference has yet to be found, but scientists all over the world have developed theories to explain the genetic basis of such behavior.
Jakob Orlebeke et al at Free University in Amsterdam postulated in 1996 that handedness is random, except in the presence of a single gene called right-shift (rs). If the wild type (rs+) allele is present, the person will develop a right-handed (RH) preference. A study was performed on 1700 Dutch families containing a pair of twins (monozygotic or dizygotic, female or male). To determine handedness, both the parents and the twins were asked, "Do you consider yourself predominantly right-handed or predominantly left-handed?" Intense statistical analysis was performed to find some indication of genetic inheritance. Of the twins born to two RH parents, 12.9% were LH. Whereas, 23.5% of the twins born to two LH parents were LH. If one parent was LH and the other was RH, the twins were 25% more likely to be LH if their mother was the LH one. These data suggest a strong genetic basis or handedness, possibly due to a gene inherited from the mother. Orlebeke also observed that twins subjected to a stressful birth or low birth weight had a higher incidence of LH than normal births or firstborn twins did.
Dr. A.J.S Klar from the Gene Regulation and Chromosome Biology Laboratory in Maryland developed a slightly different theory. Klar cites a study performed by D.C. Rife in 1940 as a more accurate account of handedness in society. Rife argued that society often encouraged naturally LH people to adopt RH habits, therefore masking the true number of LH people in society. Rife considered a person right-handed if he or she performed ten acts routinely only with the right hand: throwing a ball, using a spoon, sawing, sewing, shooting marbles, bowling, cutting with a knife, cutting with scissors, hammering and writing. Ambidextrous people in his study were considered LH. Using these criteria, Rife surveyed the families of 687 students at Ohio State University and discovered that HALF of the children from a LH x LH marriage were LH--almost double the number Orlebeke reported. Klar, using Rife's data, also hypothesized that handedness was controlled by a single gene. If children inherited one or no recessive alleles of this hypothetical gene (R/r or R/R, respectively), they would be RH. If children inherited two recessive alleles of this gene (r/r), they would randomly develop a hand preference (50% RH, 50% LH). For example, a heterozygous righty (R/r) and a homozygous lefty (r/r) have twenty children. Half of the children will be r/r. Of those 10 r/r children, 5 will be RH and 5 will be LH. Thus, 5 children would be RH*--phenotypically right-handed, but genetically equivalent to a lefty. Klar's theory explains Rife's findings perfectly, but it doesn't take into account any environmental factors. Nor does it explain the great variability in how severely LH a person may be.
So it is difficult to tell how exactly one becomes left-handed. Population genetics suggest an inheritable basis, but we can only guess until a specific gene or pathway is discovered.
References:
Klar, A.J.S., A Single Locus, RGHT, Specifies Preference for Hand Utilization in Humans, Cold Spring Harbor Symposium on Quantitative Biology, CSHL Press, (1996)
Orlebeke, J.F., Left-Handedness in Twins: Genes or Environment?, Cortex 32:479-490 (1996)
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