In articles like this one, I often read that several "genes variants are associated to a given trait". It is often added: "genetic factors explain (say) 20% of the trait variance."
The way I understand this is the following: researchers regress the trait upon the genome, find correlations between some variants and the trait and give some measure of the fit (or explained variance) like the $R^2$. Is my understanding correct?
If my understanding is correct, I am uncomfortable with the interpretation that the trait is partly genetic. Indeed, it could be due to rearing (that is, from the parents, yet not from the genes). I am aware of the practical difficulty (or impossibility) of finding a trait that we know for sure is from rearing and not genetic, but one can use models (described below) to study my concern. The basic model feature a random trait and the advanced one a trait transmitted by rearing.
The basic model runs as follows: define an arbitrary trait as the indicator of a random subgroup of the (global human) population. Would a study really fail to detect a genetic causality in this model? How common is it to find some gene variants associated to this arbitrary trait, and explaining (say) 20% of the trait variance? More formally, what is the distribution of the variance (apparently) explained, in function of the size of the arbitrary subgroup? I am sure there is a paper (or even a literature) about this: I'd like a reference to get the main insights.
Now, let's turn to the more advanced model, for which I am also seeking for references (I am sure it exists as well but have no clue how to look for it).
The model simulates genomes of the whole population among successive generations. Some trait appear at a given generation (call it t=0, although it is not the eldest generation modeled) among random individuals. The trait is not uniformly distributed at t=0, but has more chances to be found in individuals "close to the spatial location where it appear" (you can think of the trait as "listening to techno", and the location as Detroit). Suppose the trait is not genetic, in the sense that no gene variant influences the trait occurrence in one individual. Instead, the trait is transmitted through rearing environment: e.g. an individual has the trait with probability P if one of their parents has it, and with probability p<P otherwise (we could refine the assumption and say it could also be transmitted by acquaintances, or that it has more chances to be transmitted if both parents have it, but I think such refinements are not needed, and we could perhaps even simplify further and take p=0, P=1). Then, after T generations of breeding (realistically modeled), some biologists try to assess whether the trait is genetic. They will surely find some genetic correlations as (i) the trait has originally appeared in a specific location where people were relatively close genetically, (ii) the trait is transmitted by the parents, like genes, and (iii) there are many many genes, so that the probability is high that those who got the trait at t=0 share some gene variants. Now, let me recall that the trait is not genetic: for example, we could take the babies from their biological parents at t=T and have them raised by randomly drawn couples, it would be the adoptive children of techno listeners who would share the trait, and the (apparent) genetic link would be lost.
Hence my question: how do biologists know whether a trait is genetic or transmitted by rearing, despite the two seeming hard to distinguish in the advanced model? When I read a study claiming that a given trait is genetic, do they employ subtle statistical method to really prove so, or do they mean by that "genetic or reared", in the sense that the trait could fall in the case of the advanced model above? Does "explain 20% of the variance" mean that it is (probably) partly genetic?