=Segregations of the Determiners.--=How these combinations come about in this definite proportion is easily understood if the matter is expressed in terms of determiners and the possible matings tabulated (Fig. 18). If we represent the yellow determiner by Y and the green determiner by y, and likewise the determiners of roundness and angularity by R and r respectively, then the formulæ for the determiners of these two pairs of characters in the body cells (that is, in the unreduced condition) of the pure forms and of the F_{1} generation hybrids respectively are as follows: In pure round yellow peas RR YY In pure angular green peas rr yy In the hybrid Rr Yy But now in the segregation of these determiners in the germ-cells of the hybrids (generation F_{1}) the pair of determiners Rr and the pair Yy operate entirely independently of one another. Their only compulsion is that each pair be separated into the single determiners, R and r in the one case and Y and y in the other. So in the separating division which brings about this divorcement R separates from r irrespective of whether it is accompanying Y or y into the resulting daughter cell. Thus in some cases R and Y would pass into one germ-cell, in others R and y, in others r and Y, and in still others r and y, depending entirely upon the chance relations of the respective pairs to the plane of division. That is, the segregation is equally likely to be RY/ry giving gametes RY and ry, or Ry/rY giving gametes Ry and rY. [Illustration: \[Female] [Male]\ RY Ry rY ry +-------+--------+--------+--------+ RY | RRYY | RRYy | RRYY | RrYy | +-------+--------+--------+--------+ Ry | RRYy | RRyy | RrYy | Rryy | +-------+--------+--------+--------+ rY | RrYY | RrYy | rrYY | rrYy | +-------+--------+--------+--------+ ry | RrYy | Rryy | rrYy | rryy | +-------+--------+--------+--------+ (1) 1 RRYY (4) 2 RrYY (7) 1 rrYY (2) 2 RRYy (5) 4 RrYy (8) 2 rrYy (3) 1 RRyy (6) 2 Rryy (9) 1 rrYy 9:3:3:1 FIG. 18 Diagram showing the possible combinations arising in the second filial generation (F_{2}) following a cross between yellow, round (YYRR) and green, angular or wrinkled (yyrr) peas. Y, presence of factor for yellow; y, absence of such a factor; R, presence of factor for smoothness or roundness; r, absence of such a factor; [male] male; [female] female.]