An example of Mendel’s first law. Alleles of the same gene segregate independently into gametes. Plants that are homozygous for dominant round-seed shape (RR) were cross-pollinated with homozygous recessive wrinkled-seed plants (rr) to produce F1 progeny. The Punnett square analysis shows the gametes of each parent along the top and left side of the grid, from which—if the gametes are formed in equal amounts—one can predict the possible progeny and their frequency. Punnett square analysis is a common way of illustrating genetic crosses today, but was developed after Mendel’s work. In the F1 generation, the only progeny that can be produced are Rr hybrids. F1 plants were self-fertilized, and as the Punnett square analysis for the F2 generation predicts (based on the assumption that the different alleles (R and r) segregate independently into F1 generation gametes), round seeds and wrinkled seeds were produced in a 3:1 ratio.