| ♀ | 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 (F2) 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; ♀ female.
Four Kinds of Gametes in Each Sex Means Sixteen Possible Combinations.—There are, therefore, with reference to the two pairs of characters under consideration, four kinds of gametes (or mature germ-cells) produced in equal numbers in each hybrid, viz., RY, Ry, rY, and ry. That is, in the first type roundness and yellowness are associated, in the second roundness and greenness, in the third angularity (lack of roundness) and yellowness, and in the fourth angularity and greenness.
But since both males and females have these four kinds of gametes, when they are mated there will be sixteen possible combinations. These may be tabulated as in Fig. 18, opposite [p. 84].
The 9:3:3:1 Ratio.—While there are sixteen possible and equally probable combinations, these will give only nine distinct kinds because some of the matings are alike. The numbers of the various kinds of matings are as follows:
| (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 |
Since roundness (R) and yellowness (Y) are dominant to angularity (r) and greenness (y) in all combinations containing R or Y, the alternative determiners r or y would be obscured, with the result that individuals having certain of the combinations would look alike to our eye. For example, the individuals represented by numbers 1, 2, 4 and 5, since they contain dominant R and Y, would all appear round and yellow, although in reality No. 1 would be the only one of pure type (both elements homozygous) and hence the only one that would breed true in subsequent generations. The two individuals represented in No. 2 would breed true as regards shape (RR) but not color (Yy). Just the reverse is true of No. 4 since shape is heterozygous (Rr) and color homozygous (YY). The four individuals represented in No. 5 are heterozygous with regard to both elements. Thus nine individuals (1 plus 2 plus 2 plus 4 = 9) represented in Nos. 1, 2, 4 and 5 would be round and yellow, three individuals (Nos. 3 and 6) would be round and green, three (Nos. 7 and 8) would be angular and yellow, and only one (No. 9) would be angular and green. That is to say, the four classes discernible to the eye in generation F2 would be present in the ratio of 9:3:3:1.
Phenotype and Genotype.—Forms such as those represented in Nos. 1, 2, 4 and 5 which to the eye appear to be alike, regardless of their germinal constitution, are said to be of the same phenotype. Those of the same hereditary constitution, as the two individuals represented in No. 8, or the four individuals in No. 5, are said to be of the same genotype, that is, they are of identical gametic constitution.