Heredity (A): genes, alleles and Mendel’s first law

Gregor Mendel (1822–1884) crossed pea plants in the nineteenth century, because they have clear contrasting traits and can be pollinated under control. He crossed a pure tall plant with a pure short plant, and all the first generation, F₁, were tall: 100 % dominant. When he crossed two F₁ plants, the second generation, F₂, had tall and short plants in a 3 : 1 ratio. This led to his first law: the two alleles of a gene separate when gametes form, and each gamete gets one. We write P for the parents, G for the gametes, F₁ and F₂ for the generations, and × for a cross; in a Punnett square the female gametes go down the side and the male gametes across the top. A Punnett square lists all the combinations: Yy × Yy gives 1 YY : 2 Yy : 1 yy, so 3 yellow : 1 green. Careful: 3 : 1 is a probability, not a promise: a family of four plants need not have exactly one recessive plant.