When someone tells you that height is 80% heritable, does that mean:
a) 80% of the reason you are the height you are is due to genes
b) 80% of the variation within the population on the trait of height is due to variation of the genes
The answer is of course b. Unfortunately in the 5 years I’ve been blogging the conception of heritability has been rather difficult to get across, and I regularly have to browbeat readers who conflate the term with a. That is, they assume that if I say that a trait is mostly heritable I mean that its development is mostly a function of genes. In reality not only is that false, it’s incoherent. Heritability is addressing the population level correlation between phenotypic variation and genotypic variation. In other words, how well can genetic variation work as a proxy for phenotypic variation? What proportion of the phenotypic variation can be accounted for by genotypic variation? The key terms here are population level and variation (or technically, variance). We are not usually talking about individuals; and we are restricting our discussion to traits which vary within the population.
In contrast, consider the number of fingers you have on your hand. I assume you have five. How is this specified? Is it a particular diet? Do you enter into an activity which shapes your digital morphology so that it is canalized toward five fingers? Of course not. You have five fingers because there is a genetic program which specifies five fingers during your development as a fetus. There is no variation on this trait in humans aside from a few outliers, to be human is to have five fingers. Additionally, those without five fingers are assumed to be abnormal, they’re mutants, not wild type. Of course you could cut off fingers so that the environment would have an impact on the number you manifested (I know someone who had two fingers cut off in an accident at work), but this is pretty rare in the modern age. For the trait of five fingers we can say with a high degree of confidence that for any given individual it is mostly a function of genes. It isn’t incoherent to say this, there is no real variation on the trait despite a wide variation in diet and lifestyle. Even malnutrition doesn’t really alter the fact that you’ll develop five fingers, it’s a robust trait. The number of fingers you have on your hand isn’t heritable, it’s inherited.
Now on to heritability. Heritability is generally used in reference to continuous or quantitative traits. For example, height, I.Q., fingerprint ridge count and so on all exhibit continuous quantitative distributions. Heritability comes out of the tradition of statistical genetics which emerged in the late 19th century to analyze normal variation before Mendelian assumptions came to into play. While someone with three fingers is abnormal, someone who is two inches shorter than the mean is simply deviated along the normal distribution of height. Heritable traits are often subject to the independent action of numerous variables of small effect, so they naturally are subject to the central limit theorem and exhibit an approximate Gaussian distribution (the “Bell Curve”). The investigations of heritable traits is often an exploration of the nature of the variation of those traits. What proportion of the variation is genetic, environmental or an interplay between the two? The easiest way to do this is plot the values of offspring as a function of the mean of the parental values. In short if the trait is highly heritable one would see a strong linear relationship and the slope would approach the value of 1.0. As the relationship between parental and offspring values decreases the line of best fit (the regression) would start to approach 0, at which point the parental values had no bearing on the offspring values. Of course there are complications with this. For example, what about traits (e.g., height) where males and females exhibit different distributions? What about traits which exhibit strong “maternal effect” due to the impact on the fetus of the mother’s health during gestation? Of course there is the problem of gene-environment correlation between parents and offspring, e.g., the environmental heterogeneity is not random but tracks the genotypes so as to exaggerate the putative relationship between genes and outcome. Heritability is a term that needs to be used with care, nuance and subtly. In laboratory or agricultural environments where organisms can be bred in strictly controlled circumstances so as to eliminate most of the variation in non-genetic inputs one can increase heritabilities so that a maximal proportion of the variation is due to genetic variation (though this doesn’t always eliminate all the noise, as some of it occurs during development and might be the result of random infections of the mother and alterations in the fetal environment). Obviously this is a more difficult proposition when speaking of humans, necessitating the ubiquitous twin studies where as much of the shared environment is controlled as possible.
Going back to height, I noted that in developed countries it is 80% heritable. What if I told you that the heritability was lower in non-developed countries? That probably wouldn’t surprise, consider the environmental stochasticity and the greater variation in nutrient intake; it makes logical sense environmental inputs would form a greater proportion of the variation. But please note that again we are speaking about population level variations in the trait! Consider the following assertion: height is more due to environment in Third World countries than in First World countries. This might seem natural when comparing 80% heritability in the First World in height to 60% in the Third World. But the reason that heritability is so high in the First World is that sufficient nutrition exists so that it is no longer a component of variation! In other words, the greater environmental inputs result in greater heritability! Lower or more erratic environmental inputs inputs (e.g., a famine during a critical developmental period) lead to lower heritability as genetic factors cede ground to environmental parameters. Obviously when thinking about it logically the length of one’s bones are contingent upon nutritional factors, so even if heritability is high it makes no sense to say that height is “mostly genetic.” The increased heritabilities of traits as individuals age are not due to them becoming “more genetic,” rather, the non-genetic components of variation seem to drop out or attenuate over time (perhaps this is the outcome of gene-environment correlation as particular genotypes “seek out” particular environments).
Ultimately the major problem here in talking about quantitative traits and their heritable component is the imperfect mapping of statistical terminology with conventional descriptive language. That makes sense insofar as humans seem to be unequipped with much “innate statistics,” at least beyond general notions of means, medians and modes. In any case, the key point is to be cautious with language, look closely at the meaning of the terms, and remember that “common sense” interpretations of scientific terms can sometimes lead one astray.
Note: I ignored details like heritability in the “narrow sense” vs. “broad sense,” or “additive genetic variance,” to keep the post intelligible. But anyone interested in the topic should obtain a copy of D.S. Falconer’s Introduction to Quantitative Genetics. The math is at a relatively low level but hits all the major issues.