In a monohybrid cross, a 3:1 ratio indicates that both parents are heterozygous (Rr). Since wrinkled is the recessive trait in this case, a seed must be homozygous for the recessive wrinkled allele (rr, with a probability off 25%). Since round is dominant to wrinkled in this case, the round offspring have two options for their genotypes; heterozygous like the parents (Rr, with a probability of 50%) or homozygous for the dominant round allele (RR, with a probability of 25%).
