In pea plants, yellow seed is dominant over green seed. If two heterozygous yellow-seeded plants are crossed, what is the probability of green seeds?

Correct answer: One fourth

Let Y represent the dominant yellow allele and y the recessive green allele. Each heterozygous parent is Yy, so the cross is Yy × Yy. The four equally likely combinations are YY, Yy, Yy, and yy. Only yy produces green seeds because the recessive phenotype appears when both alleles are recessive. Thus 1 of 4 outcomes, or 25%, is green. The other three outcomes are yellow, so B, C, and D are incorrect.