Other analyses by our group [8] indicate that the four covariates (Sex, Age, Smoking status, and Population) are highly significant for Q4 and explain a large proportion of the variance of Q4. (Over all 200 unrelated replicates, the mean R2 is 0.787 with a range of 0.759–0.811.) We concluded that Q4 was primarily environmentally determined but that the generating model did include unlinked heritable factors.
