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.” However, no genes or measurable genetic factors (that is, entities such as alleles, tandem repeats, chromosomal inversions, etc.) are examined in deriving heritability estimates (or other quantities in quantitative genetics). Nor, as some prominent geneticists have noted (e.g., Rutter 2002, 4), does the method of analysis suggest where to look for them. Moreover, 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. In other words, the underlying factors may be heterogeneous. It could be that pairs of alleles, say, AAbbcbDDee, subject to a sequence of environmental factors, say, FghiJ, are associated, all other things being equal, with the same outcomes as alleles aabbCCDDEE subject to a sequence of environmental factors FgHiJ (Fig. 1).
Figure 1. Factors underlying a trait may be heterogeneous even when identical (or monozygotic) twins (MZT) are more similar than fraternal (dizygotic) twins (DZT). The greater similarity is indicated by the smaller size of the curly brackets. The underlying factors for two MZ pairs are indicated by upper and lower case letters for pairs of alleles (A-E) and environmental factors to which they are subject (F-J).
The possibility that the underlying genetic and environmental factors influencing development of a trait may be heterogeneous has yet to be recognized as a significant methodological concern by quantitative geneticists or by critical commentators on heritability research (e.g., Downes 2004 and references therein). It is not, of course, the case that underlying factors are always heterogeneous. Some traits are largely determined by the genes at a single locus more or less independently of the individuals’ upbringing (so called “high penetrance major genes”), for example, presence of extra digits (or polydactyly). However, the detection of such traits can be made through examination of family trees; quantitative genetics and heritability estimation need not be involved. On the other hand, there are no obvious grounds to rule out the possibility of heterogeneity in the measurable genetic and environmental factors that underlie patterns in quantitative and other complex traits, such as crop yield, height, human IQ test scores, susceptibility to heart disease, personality type, and so on. The significance of the possibility of underlying heterogeneity will be explored in future posts, as will the challenges of getting others to appreciate the concept.
Downes, S.M.: 2004, Heredity and Heritability, in E. N. Zalta (ed.), The Stanford Encyclopedia of Philosophy, (http://plato.stanford.edu/entries/heredity/ (viewed 11 May 2006).
Rutter, M.: 2002, Nature, Nurture, and Development: From Evangelism through Science toward Policy and Practice, Child Development 73(1), 1-21.