ses 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.

[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