That is, one fourth are pure black (BB), one fourth pure white (WW), and the remaining half are hybrids, black and white (BW). The pure blacks again form germ cells, all possessing the determiner for blackness; the pure whites form germ cells all lacking the determiner for blackness; the hybrid blues produce again equal numbers of germ cells possessing and lacking the determiner for blackness. The relation of the germ cells and the organisms forming them and developing from them is shown in the diagram in Fig. 9.
In the more common cases where the phenomenon of dominance appears, as in the guinea pig, this is explained by saying that here a single determiner for blackness is somehow sufficient to produce the color. In such cases the black color observed may result either from a single (BW) or from a double (BB) black determiner in the germ which forms the organism. Only when the black determiner is entirely absent (WW) does the white color appear in the developed organism and the individual is then said to exhibit the recessive characteristic.
Fig. 9.—Diagram illustrating the relation of the germ cells in a simple case of Mendelian heredity, such as that of color as shown in Figs. 7 and 8. The spaces between the large circles represent the bodies of the individuals while the small circles within each represent the germ cells formed by those individuals. P, parental generation; each individual forms a single kind of germ cells. G. F1, germs of the first filial or hybrid generation, each composed of two different kinds of germ cells, one from each parent. F1, individuals of the first filial or hybrid generation, developed from G. F1. Each member of this generation forms two kinds of germ cells in approximately equal numbers. G. C. F1, germ cells of F1, showing possible combinations resulting from the mating of two members of F1. Each of these combinations occurs with equal probability. G. F2, germs of second filial generation resulting from the above random combinations. F2, individuals of second filial generation. Each now forms germ cells like those which constituted its own germ.
Another possible type of mating is that between a member of a pure race, either dominant or recessive, and a hybrid individual. This form of mating is very common in some of the pedigrees that we shall examine later. The results of such a mating, first between a hybrid and a recessive individual can be most easily described by considering a cross between black and white forms and expressing the result algebraically.
| Germ cells of first parent (white or recessive) | W | + | W | ||
| Germ cells of second parent (hybrid) | B | + | W | ||
| ————————— | |||||
| BW | + | BW | |||
| WW | + | WW | |||
| ———————————————— | |||||
| 2BW | + | 2WW | |||
That is, returning to the example of the Andalusian fowls, the progeny will be one half hybrid blues and one half whites—no black at all. If the cross had been between black hybrid guinea pigs and white recessive specimens the result would have been half hybrid blacks and half pure whites.
Or supposing the mating to have occurred between the pure dominant (black) and the hybrid the result would have been, in the fowls half pure black and half hybrid blue; in the guinea pig all the progeny would have been black, half pure blacks and half hybrid blacks.
| Germ cells of first parent (black or dominant) | B | + | B | ||
| Germ cells of second parent (hybrid) | B | + | W | ||
| ————————— | |||||
| BB | + | BB | |||
| BW | + | BW | |||
| ———————————————— | |||||
| 2BB | + | 2BW | |||