Mendel therefore concluded that individual germ cells are always pure with respect to any pair of contrasting characters, even though those germ cells have come from hybrids in which the contrasting characters are mixed. A single germ cell can carry the factors, or causes, for red or white flowers, for green seeds or yellow seeds, for tall stem or short stem, etc., but not for both pairs of these contrasting characters. The hybrids formed by crossing white and red four o'clocks carry the factors for both white and red, but the individual germ cells formed by such a hybrid carry the factors for white or red, but not for both; these factors segregate or separate in the formation of the germ cells so that one half of all the germ cells formed carry the factor for white and the other half that for red.
This is the most important part of Mendel's Law—the central doctrine from which all other of his conclusions radiate. It explains not only the segregation of dominant and recessive characters from a hybrid in which both are present, but also the relative numbers of pure dominants, pure recessives, and mixed dominant-recessives in each generation. For if all germ cells are pure with respect to any particular character the hybrid offspring of any two parents with contrasting characters will produce in equal numbers two classes of germ cells, one bearing the
Fig. 52. Diagram of Mendelian Inheritance, in Which the Individual is Represented by the Large Circle, the Germ Cells by the Small Ones, Dominants Being Shaded and Recessives White. a, Pure dominant x pure recessive = all dominant-recessives, b, Dominant-recessive x dominant-recessive = 1 pure dominant: 2 dominant-recessives: 1 pure recessive, c. Dominant-recessive x pure dominant = 2 pure dominant: 2 dominant-recessive, d, Dominant-recessive x pure recessive = 2 dominant-recessive: 2 pure recessive.
dominant and the other the recessive factor, and the chance combination of these two classes of male and female gametes will yield on the average one union of dominant with dominant, two unions of dominant with recessive and one union of recessive with recessive, thus producing the typical Mendelian ratio, 1DD : 2D(R) : 1RR, as shown in the accompanying diagram (Fig. 52, A, B).
