The following shows an example of the \chi ^2 method applied to experiments performed by the biologist Gregor Mendel (July 20, 1822 – January 6, 1884).
In one of his famous experiments (sometimes called Mendel's First Experiment) he crossed strains of garden peas with yellow and green seeds. He then allowed the F_1 generation (which were all yellow) to self-fertilize, collected the resulting seeds and grew them all. In the resulting F_2 generation, he found that the ratio yellow:green was 3:1.