Kernel color in corn is determined by two genes (A and B), with two alleles of each (A1, A2, B1, B2). Each 1 allele contributes a single dose of color, and each 2 allele contributes no color. The greater the number of 1 alleles, the darker the color. If double heterozygotes are selfed (A1A2 x B1B2), what proportion of the progeny will have exactly three doses of color?