Why you be short or tall (well, a little bit)

Curious about height? Check out this new paper in Nature Genetics, A common variant of HMGA2 is associated with adult and childhood height in the general population. Nature News has a nice report for public consumption.

Last week when I posted about heritable traits I used height as an illustrative example. The reasoning was pretty simple, it's a rather concrete phenotype which most humans have an intuitive grasp of in terms of the range of variation, and it also happens to exhibit high heritability in developed populations; on the order of 0.90, i.e., 90% of the variation in the population is due to variation in genes. As a continuous quantitative trait the variation in normal human height is generated by the combined additive and independent action of many genes (at least in theory).¹ Of course if you divide 100% by many factors each factor has only a small impact, so it can be quite difficult to ascertain exactly which regions of the genome has a significant effect on a trait's variation within a population. And so it is with height. These researchers used genome-wide association to detect a locus of significant effect on height: two copies of the 'tall' allele of HMGA2 result in 1 extra centimeter vs. those with two copies of the 'short' allele, and in an additive fashion one copy of each results in a average effect exactly equidistant in value. Nevertheless, the frequency of the 'tall' and 'short' allele at this locus explains only ~0.3% of the within population variation! A little something is better than nothing; and the theory is that combined research of small effect can result in a knowledge base of great extent.

Note that this finding is in regards to a single nucleotide polymorphism (SNP), one allele has a "T" base pair while the other has a "C." This one "letter" change resulted in a change of function that is responsible for a small, but significant, proportion of the variation in human height. It wouldn't surprise me if other sources of genetic variation, such as copy number, don't end up to be just as important for height. Though some researchers make the claim that the normal range of human height is invariant throughout populations, I am skeptical of this a priori. Large mammals tend to exhibit differences between populations in morphology from region to region. For example, Bergmann's Rule states that members of a species in colder climates will have larger body masses as well as lower surface area to volume ratios. The adaptive rationale for this is pretty obvious when it comes to metabolic management. So why would height vary between populations? We know that it is unlikely to be strong directional selection, this process would exhaust the genetic variation which quantitative traits are generated by. Rather, weak (and possibly balancing) selective forces are likely at work within and between populations. One could posit that height is simply the outcome of neutral evolution on a quantitative trait; but a long line of research suggests fitness boosts, up to a point, for taller males. My own hunch is that within human populations larger size is preferred ceteris paribus, but with the shift to agriculture and a low protein diet smaller size could be a benefit in sustaining energy in the face of endemic malnutrition. It doesn't pay to be sexy if you're dead. So the low average height of some high density agricultural populations isn't simply environmental, the reduction of expected resource allotment selected for individuals who were less on the margins for their metabolic needs. In other words, short people.

1 - Dominance and epistatic effects can result in deviations from a linear model; though often enough the deviation is small enough to allow the use of additive & independent models.