In a genetic cross between two heterozygous pea plants (Ss), what fraction of the offspring will have spherical seeds?

In a genetic cross between two heterozygous pea plants, represented as Ss, we are looking at the inheritance of a trait where S denotes the dominant allele for spherical seeds and s represents the recessive allele for wrinkled seeds. When two heterozygous plants are crossed, the possible combinations of alleles in the offspring can be determined using a Punnett square.

The combinations from the Ss x Ss cross will result in four potential genotypes for the offspring:

1. SS (homozygous dominant)

2. Ss (heterozygous)

3. Ss (heterozygous)

4. ss (homozygous recessive)

When we analyze these genotypes regarding their phenotype, both the SS and Ss genotypes display the spherical seed trait since the spherical seed allele (S) is dominant. Therefore, among the four possible combinations (genotypes), three of them (SS and two Ss) will have spherical seeds.

Thus, the fraction of the offspring that will exhibit spherical seeds is 3 out of 4, resulting in the correct answer being 3/4. This understanding of dominance and the application of a Punnett square facilitates the prediction of phenotypic ratios in genetic crosses.