Re: Is being left or right handed linked to genentics ?

Date: Mon Jan 12 17:58:43 1998
Posted By: Christopher Carlson, Grad student Genetics
Area of science: Genetics
ID: 883853578.Ge

	If you weren't able to find any info re: inheritance of handedness 
using a library search engine, either you used the wrong search engine, or 
you asked the wrong question.  Here's one of the better sites for such 
searches:  As for the question to ask, 
I looked up all review articles published since 1991 with the keyword 
"handedness" and found ~40 of them.  Only 12 were relevant, but it wasn't 
hard to figure out which they were.

	Coincidentally, one of the bigger names in handedness research also 
hails from Canada: Stanley Coren is at UBC.  Some of my reply paraphrases 
the article by Coren and Halpern in the Psychological Bulletin, 109(1), pp. 
90-106.  It is from 1991, and received quite a bit of media time because 
the basic assertion made was that the decline of left handers as a percent 
of the population as stratified by age is due to increased mortality.  
Translation, left handers die at younger ages because it's a right hander 
biased world.  The debate still rages, but several groups have found 
similar results in other populations.

	However, you didn't ask about the risks associated with being left 
handed.  What you asked was whether handedness is linked to genetics.  
Well, it looks like handedness is heritable but it's not a simple Mendelian 
trait.  About 13% of the human population is left handed.  This ratio is 
remarkably similar throughout the world, and has led several groups to 
suggest that it is maintained by selection for a locus with heterozygote 
advantage (higher fitness in heterozygotes) at which there are two alleles, 
one for dextral preference (right handedness) and the other for random 
dextral/sinistral preference.  This model is still rather controversial, 
but it does offer a way to explain the relatively constant frequency of 
left handedness globally.

	Another way to look at the genetics of handedness is to ask whether it 
runs in families.  One measure of this is relative risk, defined as the 
ratio between the risk of left handedness given a left handed relative and 
the general risk of left handedness in the population.  I calculated the 
following relative risks from the data presented in Corballis et al., 
American Journal of Medical Genetics 67:pp. 50-52.  The relative risk that 
siblings of left handers will be left handed is ~1.6 (they are 1.6 times 
more likely to be left handed than the general population, or siblings of 
lefties are ~20% left handed).  Interestingly, the relative risk to 
offspring of lefties is ~2.1.  The fact that this is higher than the 
relative risk to sibs suggests that there may be something environmental 
involved: perhaps children who have ambidextrous tendencies are influenced 
by the dexterity of their parent. However, both of these numbers suggest a 
significant heritable component of handedness based upon clustering of the 
trait in families. 

	An alternative explanation that has been offered is that left 
handedness correlates with distress during childbirth.  Left handed mothers 
tend to have a higher frequency of distressed births, and this could 
conceivably account for the increased relative risk to offspring and even 
the increased relative risk to sibs.  I don't really hold with this notion, 
but i wasn't able to find any studies which conclusively refute it.

	Chris Carlson


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