Proportion of variation attributable to a QTN (QTL)

Question

[DiegoV.O.]

Hello,

I'm doing a simple simulation of quantitative traits. I came across this equation from this paper:

Where Yi is the phenotype of the i individual, pi is the proportion of variation explained by the QTN (quantitative trait nucleotide), Zi is a normal random deviate and Qa and Qb are the allelic states of the QTN.

I've been trying to figure out where does that equation come from, but without success so far. Unfortunately, the paper does not cite any reference.

I'd appreciate any help about the origin of this equation.

PS: I am familiar with the dominance and additive genetic variance equations; as well as other related concepts.