[Illustration: 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.