No, actually, genetic studies of both milk production and IQ show them to be mainly linear.
What kind of study do you think shows IQ to be mainly linear?
I would guess that you confuse assumptions that the researchers behind a study make to reduce the amount of factors with finding of the study.
There are decades of studies of the heritability of IQ. Some of them measure H², which is full heritability and some of them measure h², "narrow sense heritability"; and some measure both. Narrow sense heritability is the linear part, a lower bound for the full broad sense heritability. A typical estimate of the nonlinear contribution is H²-h²=10%. In neither case do they make any assumptions about the genetic structure. Often they make assumptions about the relation between genes and environment, but they never assume linear genetics. Measuring ...
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