Skin color in humans is an example of a phenotypic trait that is determined by polygenic inheritance. three separate gene loci determine skin color (p, q and r), where each gene locus has an additive effect on skin color and the capital letters represent incompletely dominant alleles producing dark skin. what fraction of the children of a medium-toned (ppqqrr) mother and medium-toned (ppqqrr) father would exhibit the lightest possible skin color(genotype: ppqqrr)?

1/64 is the answer. Polygenic inheritance occurs when the characteristics is under the control of two or more genes. In the presence of large number of genes it is very hard to differentiate the effect of each individual gene in accordance to the Mendelian rules. 

Example, There are 3 genes let us designate with X, Y, Z. Each goes in two alleles. One express the trait (capital) and the other cannot express (lower case). These can have an additive effect. xx cannot contribute to trait and Xx genotype cane contribute to trait and AA would express more of the trait. The same thing would happen to other genes Y, Z.