The possibility of underlying heterogeneity makes heritability studies even less informative than, as prominent geneticists have noted (e.g., Rutter 2002, 4), the method of data analysis not suggesting where to look for the underlying genetic factors that contribute to heritability. Here I comment on the difficulty of translating from statistical analyses of data on traits to hypotheses about the measurable genetic or environmental factors involved in the development of the traits.
First recall what heritability means:
Consider claims that some human trait, say IQ test score at age 18, show high heritability. These claims can be derived from analysis of data from relatives. For example, the similarity of pairs of monozygotic twins (which share all their genes) can be compared with the similarity of pairs of dizygotic twins (which do not share all their genes). The more that the former quantity exceeds the latter, the higher is the trait’s heritability. Researchers and commentators often describe such comparisons as showing how much a trait is “heritable” or “genetic.” (from previous post)
Now the possibility of underlying heterogeneity is that
even if the similarity among twins or a set of close relatives is associated with similarity of (yet-to-be-identified) genetic factors, the factors may not be the same from one set of relatives to the next, or from one environment to the next (from previous post).
Imagine now that I revealed that the trait in this picture was body length and the twin pairs in this figure came from different species. You might ask why do heritability studies on pairs from different species–what could we do with the answer? But note, there is nothing in the method of estimating heritability that takes into account whether the pairs came from the same species or not. Indeed, the species could be Homo sapiens in Siberia, Pseudechis porphyriacus in Australia, and Sorghum bicolor in Asia. You would not expect that the genetic factors underlying the high heritability of body length in humans, red-bellied black snakes, and sorghum were the same. I suspect that you wouldn’t take high heritability of body length from this study as an indication that this trait is a good candidate for “molecular research to identify the specific genetic factors involved” (see another previous post). The reason—you would expect that the “even if the similarity among twins or a set of close relatives is associated with similarity of (yet-to-be-identified) genetic factors, the factors [would] not be the same from one set of relatives to the next.” But how closely related do the twin pairs have to get before you stop being concerned about the possibility of underlying heterogeneity? If the twin pairs were all from the same order would we use heritability as a guide to undertake molecular research? From the same genus? Same species? Same population? The methods of heritability studies do not tell us what degree of relatedness among the twin pairs (as against within the twin pairs) is close enough that the possibility of underlying heterogeneity disappears. Which leads us back to the question of a previous post, “What can researchers do on the basis of knowing a trait’s heritability if the genetic and environmental factors underlying the observed trait are heterogeneous?”
More in the future on the challenges of getting others to appreciate the significance of the possibility of underlying heterogeneity. For now, let me note that the fact that similarity of twin pairs is not the only way to estimate heritability is not a pertinent objection. If heritability as estimated by the other means can also be estimated by similarity of twin pairs, the conceptual point made in this post must be relevant to those methods as well.