He is probably measuring something called "narrow-sense heritability", which just includes just additive genetic effects (Gene A + Gene B = Sum(A, B)) and exclu

He is probably measuring something called "narrow-sense heritability", which just includes just additive genetic effects (Gene A + Gene B = Sum(A, B)) and excludes other things such as gene interaction. Most of the statistical techniques used to estimate the narrow-sense heritability assume environmental and other, non-additive, genetic effects follow a Gaussian. Your example clearly exposes a situation where these studies may not be powerful. The inability to explain heritability as more than a "sum of the parts" is a problem which I'm not familiar with a solution for at the genome scale.