Genetics Primer, 2: the role of chance

As we introduced with eye color, there may be random variation in the expression of a trait. Interestingly, a current theory of handedness suggests that this is exactly what happens for right handed versus left handed. Let's call the gene for Right handedness R, and the other version of this gene r. So, following our B/b diagram, we would suggest that RR or Rr people are right handed, and rr people are left handed. This works at first because we know that there is a genetic component to handedness in families.

But when you look in detail for that kind of inheritance, it doesn't work. There are still fewer left handed people than there should be, and there are some right handed people with left handed parents. What DOES fit the data is that "rr" isn't determinate of left handedness, it simply creates a random chance of either.

So the absence of R isn't "left", it's "either left or right determined at random". And again, potentially subject to modifiers in the environment, in the genome, and so on. Thus in two identical siblings, both rr, one can be left handed and one can be right handed simply by chance. There's still a genetic component, and they are still genetically identical, even though the outcome (the phenotype, in scientific terms) differs. Cool, isn't it?

One model suggests that this sort of characteristic is determined by a molecular mechanism to skew the distribution of daughter DNA molecules. Interestingly, there is a correlation with other asymmetrical characteristics, including left-right asymmetry in the brain, and the directionality of hair whorls on the crown of our heads. What's correlated is the asymmetry, not which way it worked out: that is, counterclockwise hair whorls are equally split between right and left handed people. So the counterclockwise whorl didn't influence your handedness; what they share is that both were determined randomly.

Oh, and the counterclockwise hair whorls may be more prevalent in gay men. Does this mean that if you are counterclockwise in hair, you are gay? No, nor does it mean that all gay men have counterclockwise hair whorls. But it might suggest there is some asymmetric component to sexuality, at least in men.

So you can see even in apparently simple human traits (eye color, handedness) there is a lot of complexity in how our genes are "read" and how they interact with and influence each other. And, we are not fully determined by our genetic information. Rather, genetics provides a palette but each of us is a unique painting that is a combination of genetics, environment, and chance. So, for complicated traits like behavior, you can predict that there will be a wide range of behaviors and they will be highly variable. All of which is perfectly "normal".



To read this entire series in order, visit the Genetics Page.