Fig. 21. Cross between yellow-round and green-wrinkled peas, giving the 9: 3: 3: 1 ratio in F2.
We can, as it were, take the characters of one organism and recombine them with those
of a different organism. We can explain this result as due to the assortment of factors for these characters in the germ cells according to a definite law.
Fig. 22. Diagram to show the history of the factor pairs yellow-green and round-wrinkled of the cross in Fig. 21.
As a second illustration let me take the
classic case of the combs of fowls. If a bird with a rose comb is bred to one with a pea comb (fig. 23), the offspring have a comb different from either. It is called a walnut comb. If two such individuals are bred they give 9 walnut, 3 rose, 3 pea, 1 single. This proportion shows that the grandparental types differed in respect to two pairs of characters.
Fig. 23. Cross between pea and rose combed fowls. (Charts of Baur and Goldschmidt.)
A fourth case is shown in the fruit fly, where an ebony fly with long wings is mated to a grey fly with vestigial wings (fig. 24). The