Gregor Mendel worked out the rules of inheritance by counting pea seeds, and this page reproduces his dihybrid experiment as a Punnett square you can see: yellow or green seed colour and round or wrinkled seed shape, drawn as peas in every cell. The cross opens on two plants heterozygous for both traits and produces the 9:3:3:1 ratio Mendel counted in 1865.
Seed colour is controlled by one gene: the yellow allele is dominant, so a plant with one yellow and one green allele produces yellow seeds. Seed shape is a second gene, with round dominant over wrinkled. In the calculator the first gene stands for colour (A yellow, a green) and the second for shape (B round, b wrinkled), so a plant heterozygous for both is written AaBb. Switch the display to Peas and each cell shows the seed itself: smooth yellow, smooth green, wrinkled yellow or wrinkled green, with the genotype letters in the corner.
Each AaBb plant makes four kinds of pollen or egg, AB, Ab, aB and ab, so the square has sixteen cells. Nine of them contain at least one yellow and one round allele and show as yellow round seeds; three are yellow wrinkled, three green round and one green wrinkled. Mendel’s actual count from this cross was 315 : 108 : 101 : 32, and the calculator’s chi-square test confirms that those numbers fit 9:3:3:1. The Phenotype view merges the cells into these four groups, while the Genotype view shows the nine underlying genotypes and their 1:2:1:2:4:2:1:2:1 count.
AABB × aabb, a pure yellow round plant crossed with a pure green wrinkled one. Every seed is yellow and round, exactly as Mendel saw in his first generation.Aa × Aa, which gives three yellow seeds to one green. Mendel’s count here was 6,022 yellow to 2,001 green.Mendel was fortunate in his choice of traits: the genes he followed sit on different chromosomes or far enough apart to assort independently, which is why the 9:3:3:1 ratio holds. The peas here also assume complete dominance. Switch the mode to Incomplete and the heterozygous cells become yellow-green and slightly wrinkled seeds, a useful reminder that real traits do not always behave like Mendel’s. For the same cross without the pictures, see the dihybrid cross calculator.
Seed colour, with yellow dominant over green, and seed shape, with round dominant over wrinkled. The Peas display draws each cell as the seed the offspring would produce.
Crossing plants heterozygous for both seed traits gave 315 yellow round, 108 yellow wrinkled, 101 green round and 32 green wrinkled seeds, close to 9:3:3:1, which showed that the two traits are inherited independently.
Peas have clear either-or traits, grow quickly, self-pollinate unless deliberately crossed, and produce many seeds, so ratios could be counted from hundreds of offspring in one season.
Seed shape, seed colour, flower colour, pod shape, pod colour, flower position and stem length. Each has one dominant and one recessive form.
Yes. Open Trait names under the parents and type your own names, for example Purple and White or Tall and Short; the analysis and the tap readout then use those words.