If A denotes one of the two constant traits, for example, the dominating one, a the recessive, and the Aa the hybrid form in which both are united, then the expression: gives the series for the progeny of plants hybrid in a pair of differing traits.
Gregor Mendel, Alain F. Corcos, Floyd V. Monaghan (1993). “Gregor Mendel's Experiments on Plant Hybrids: A Guided Study”, p.96, Rutgers University Press
