The answer is rr. In a monohybrid cross, the phenotypic ratio is 3:1 for
dominant trait to recessive trait. Since the ration is 3:1 for round
seeds to wrinkled seeds, we can conclude that round seed is the dominant
trait. Let R be a dominant allele for round seed and r be a recessive
allele for wrinkled seed. The genotypes will be: RR (dominant homozygote
with round seeds), Rr (heterozygote with round seeds), and rr
(recessive homozygote with wrinkled seeds. Thus, only genotype rr is the
genotype of the wrinkled seeds.
