Mendel: Experiment 1

Legend:

This drawing illustrates Mendel's "Experiment 1",  in which he demonstrated his concept of heredity in the mating of pea plants.

Mendel suspected that heredity depended on contributions from both parents and that specific characteristics from each parent were passed on, rather than being blended together in the offspring.

A parent homozygous for the spherical seed allele is crossed with a parent homozygous for the wrinkled seed allele. Each parent makes gametes of only one kind, either S or s, and these combine at fertilization to form plants that all have the genotype Ss and the spherical seed phenotype.

When the F1 plants self-pollinate they produce two kinds of eggs, S and s, and the same two types of male sex cells. These combine randomly in four different ways to form F2 plants.
Three of the four possible combinations produce "spherical seed" phenotype, and the fourth produces "wrinkled seed" phenotype, so that the observed ratio is 3:1.

The wrinkled seed phenotypes can only correspond to the "ss" genotype. The spherical seed phenotype corresponds, in one case to the "SS" genotype, and in the two others to the "Ss" genotype. Two of the offsprings are homozygous (SS or ss), the two others are heterozygous (Ss).

The fact that the spherical seed phenotype is observed with the genotypes "SS" or "Ss" can only be explained if the character "spherical seed" is dominant and the character "wrinkled seed" recessive.

The illustration at the bottom of the page, called a Punnett Square, is a handy device for keeping track of the different ways in which gametes combine at fertilization.