one, and since the number of types of offspring increases greatly when more than one or two characters are considered at the same time, it is customary to deal simultaneously with only one or two characters of hybrids, even though the parents may have differed in many characters.
Fig. 55. Trihybrid Diagram Showing Results of Crossing Peas Having round-yellow seeds and tall stem (RYT) with peas having wrinkled green-seeds and Dwarf Stem (WGD). Eight types of germ cells result from such a hybrid, as shown in the ♂ gametes above the square and the ♀ ones to the left of it, and the possible combinations (genotypes) of these ♂ and ♀ gametes are shown in the 64 small squares of which only 1 is pure dominant (upper left corner), 1 pure recessive (lower right corner) and 8 homozygotes (in diagonal line between these corners). The relative numbers of the different phenotypes are 27 RYT : 9 RYD : 9 RGT : 9 WYT : 3 RGD : 3 WYD : 3 WGT : 1 WGD.
3. Inheritance Formulæ.—Mendel represented the hereditary constitution of the plants used in his experiments by letters employed as symbols, dominant characters being represented by capitals and recessives by small letters. The seven contrasting characters of his peas could be represented as follows:
| Seeds, round (A), or wrinkled (a); yellow (B); or green (b); with gray seed coats (C), or white seed coats (c). |
| Pods, green (D), or yellow (d); inflated (E), or constricted (e). |
| Habit, tall (F), or dwarf (f). |
| Flowers, axial (G), or terminal (g). |
