50 whys to look for genes: 3. High heritability => genetic influences > environmental

If heritability is high for a given trait, it might seem that genetic influences outweigh environmental and resources for research to understand the trait are better devoted to looking for the genes rather than the environmental influences.

There are many complications to this line of thinking, including the “it might seem” reasoning and the rise of a new concept of heritability that shares the same name as heritability in the classical sense (as collated in this table of estimates).  These issues are discussed on pages 31-32 and 124-5 respectively of Nature-Nurture? No, but the complication to be discussed here concerns underlying heterogeneity.  From p. 19-20 of Nature-Nurture? No:

Claims that some human trait, say, IQ test score at age 18, shows high heritability derive from an analysis of data from relatives. For example, the similarity of pairs of monozygotic twins (who share all their genes) can be compared with the similarity of pairs of dizygotic twins (who do not share all their genes). The more that the former similarity exceeds the latter, the higher the trait’s heritability. Researchers and commentators often describe such calculations as showing how much a trait is heritable or genetic. However, no genes or measurable genetic factors (such as alleles, tandem repeats, or chromosomal inversions) are examined in deriving heritability estimates, nor does the method of analysis suggest where to look for them.

Moreover, even if the similarity between twins or a set of close relatives is associated with the 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 (see Figure A.2 for the case of human twins where both members of each pair are raised in the same household). If the genetic and environmental factors underlying the observed trait are heterogeneous, what can researchers do on the basis of a heritability estimate?…

twins heterogeneity


Figure A.2. Factors underlying a trait may be heterogeneous even when identical (monozygotic) twins raised together (MZT) are more similar than fraternal (dizygotic) twins raised together (DZT). The greater similarity is indicated here by smaller size of the curly brackets. The underlying factors for two MZT pairs are indicated by upper- and lowercase letters for pairs of alleles (A-E) and the environmental factors to which they are subject (F-J).

(Introduction to this series of posts)

3 thoughts on “50 whys to look for genes: 3. High heritability => genetic influences > environmental

  1. Pingback: 50 whys to look for genes: 11. Eliminate the distinction between familial and hereditary cancers | Intersecting Processes

  2. Pingback: 50 whys to look for genes: 20. If it is passed down, there’s nothing we can do (could have done) | Intersecting Processes

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