 But this equation still oversimplifies the situation and requires more refining to deal with quantitative traits. Variance in phenotype can result from gene interaction effects, or epistatic variance, \(V_{I}\). This occurs when alleles at one locus have an effect on the phenotype that is dependent upon alleles at one or more other loci. Further, there may be a contribution to phenotypic variance from gene/environment interaction, \(V_{G\times E}\) (See Tabery 2014 for a detailed discussion of gene/environment interaction.). This occurs when the effect of the environment on the phenotype differs between genotypes. Finally, \(V_P\) can be affected by non-random correlations between genotypes and environments referred to as gene-environment covariation, \(\COV(G,E)\). For example, if plants with a genotype that tends to produce large plants also select nutrient- rich environments, by root growth or seed dispersal, and plants with a genotype that tends to produce small plants also select nutrient- poor environments, the variance in height would be increased. If the relation were switched the variance would decrease (Futuyma 1998). Factoring all the above in we now have the following: