In genetics, the dash symbol (-) is a "wild card" that stands for either the dominant allele or the recessive allele; for example, W- means the individual has either the genotype WW or Ww. In summer squash, genotypes W- G- and W- gg are white, ww G- are yellow, and ww gg are green. W and G show independent assortment. What is the ratio of white : yellow : green among progeny of the cross Ww Gg × Ww Gg?

According to Mendel's principles, a dihybrid cross between two heterozygous summer squash parents (WwGg X WwGg) should yield offsprings, 12 with white, 3 with yellow and 1 with green color phenotype. The classic Mendelian phenotypic ratio for dihybrid cross is 9:3:3:1. In this case, genotypes W- G- and W- gg produce white color phenotype. This is because we have 12: 3: 1 phenotypic ratio in which the alleles of two different genes assort independently into gametes.

Dihybrid cross for heterozygous summer squash:

WG Wg wG wg

WG WWGG WWGg WwGG WwGg

Wg WWGg WWgg WwGg Wwgg

wG WwGG WwGg wwGG wwGg

wg WwGg Wwgg wwGg wwgg

Genotypes:

W-G- and W- gg = white = 12

ww G- = yellow = 3

ww gg = green = 1