Two loci affecting seed colour, two alleles each. With two loci and two alleles, we obtain five different phenotypic classes. The more gene loci, the more phenotypic classes there will be. For five plus and five minus, though, there are 51 possible genotypes. If plot out numbers of genotypes, gives normal dist of phenotypes.
Extremes arise from rare and unfortunate combinations of genotypes.
Universally accepted that several genes influence quantitative characters. Important to note that in all characters we are looking only at phenotypes, not directly at genes. Many phenotypes are also affected by the environment. This is because different genotypes may have diff Norms of Reaction. Classic eg - neural tube defects.