Figure 1

Mendel's diagrammatic explanation for the formation of the F2 population of plants produced by self-fertilisation of his F1 hybrids. Mendel proposed that F1 hybrids (Aa) contained a dominant trait (A) that was displayed and a recessive trait (a) that was not displayed. Self-fertilisation of F1 hybrids (Aa) then involved segregation of the component traits (A) and (a) into individual male pollen and female germinal cells, as shown in his diagram. Mendel proposed that if a male pollen cell carrying a trait (A) fertilised a female germinal cell carrying the same trait (A), the progeny would display trait (A). He used the analogous argument for the generation of progeny bearing trait (a). Only if male and female sex cells carried differing forms of a given trait (A or a but not both) would the progeny be hybrids (Aa). Thus random recombination of the segregated traits during self-fertilisation of hybrids would yield (on average) the F2 population of plants represented by the trait series (A + 2Aa + a) shown below Mendel's original diagram.