Punnett squares

The possible genotypes and phenotypes of the offspring can be determined using a Punnett square, a grid that shows the possible combinations of alleles that can result at fertilisation.

The Punnett square below shows the expected genotypes of the offspring of parent pea plants that both have the genotype Rr. Each of their gametes only contains one allele of the gene for seed shape.

Genotypes of female gametesGenotypes of female gametes
Genotype of male gametesRr
RRRRr
rRrrr

The Punnet square shows the possible genotypes of the offspring. Three quarters of the possible combinations of alleles (one RR and two Rr) will give rise to plants that produce seeds with the dominant round phenotype.

Only one combination of the male and females alleles (rr) will give rise to plants that produce seeds with the recessive, wrinkled phenotype.

This means the expected ratio of offspring plants that produce round seeds to plants that produce wrinkled seeds will be 3:1.

However, predicted phenotype ratios among offspring are not always achieved. This can be for a variety of reasons, for example the sample size being too small or the ratios based on fertilisation (which is a random process).