Answer;
The correct answer would be Rr.
In a monohybrid cross, the phenotype ratio of 3:1 is obtained only when both the parents are heterozygous for the trait.
In this condition, the heterozygous genotype would be Rr. The phenotype of both the parents would be round seeds (R) as the round is a dominant trait.
The cross of these parents would produce offspring with three types of genotypes RR, Rr, and rr in ratio 1:2:1.
Thus, the phenotypic ratio would come out to be 3 (round seed):1 (wrinkled seed).
