In pea plants, spherical seeds (S) are dominant to dented seeds (s). If two plants heterozygous for seed shape (Ss x Ss) are crossed, what fraction of offspring will have spherical seeds?

In pea plants, spherical seeds (S) are dominant to dented seeds (s). If two plants heterozygous for seed shape (Ss x Ss) are crossed, what fraction of offspring will have spherical seeds?

• A. 25%
• B. 50%
• C. 75%
• D. 100%

Answer: (C)

Spherical seeds appear whenever at least one S allele is present, because S is dominant over s. Crossing two heterozygotes (Ss x Ss) produces four equally likely genotype combinations: SS, Ss, Ss, and ss. The spherical phenotype corresponds to SS and Ss, which accounts for three of the four possibilities. So three-fourths of the offspring, or 75%, will have spherical seeds. The remaining one-fourth, ss, will be dented.

Spherical seeds appear whenever at least one S allele is present, because S is dominant over s. Crossing two heterozygotes (Ss x Ss) produces four equally likely genotype combinations: SS, Ss, Ss, and ss. The spherical phenotype corresponds to SS and Ss, which accounts for three of the four possibilities. So three-fourths of the offspring, or 75%, will have spherical seeds. The remaining one-fourth, ss, will be dented.