In P plants the allele for round seeds (R) is dominant and the allele for wrinkled seeds (r) is recessive. Under what conditions would we observe wrinkled seeds in a plant

A dominant trait can be described as a trait which arises from a dominant allele. A dominant allele is the one which suppresses the effect of a recessive allele. A recessive trait arises from a recessive allele. For a recessive trait to occur, both the alleles of a gene should be recessive.

For a dominant trait to occur, the gene could have homozygous alleles i. e both the alleles would be the dominant ones or the alleles could be heterozygous i. e one dominant allele and one recessive allele.

Hence, to observe wrinkled seeds both the alleles should be recessive i. e rr.