all we call the
dominant? If the ebony, then in the second generation we count three
ebonies to one sooty, putting the hybrids with the ebonies. If the dominant
is the sooty then we count three sooties to one ebony, putting the hybrids
with the sooties. The important fact to find out is whether there actually
exist three classes in the second generation. This can be ascertained even
when, as in this case, there is a perfectly graded series from one end to
the other, by testing out individually enough of the flies to show that
one-fourth of them never produce any descendants but ebonies, one-fourth
never any but sooties, and one-half of them give rise to both ebony and
sooty.

[Illustration: FIG. 20. Cross between two allelomorphic races of
Drosophila, sooty and ebony, that give a completely graded series in F_2.]

MENDEL'S SECOND DISCOVERY--INDEPENDENT ASSORTMENT

Besides his discovery that there are pairs of characters that disjoin, as
it were, in the germ cells of the hybrid (law of segregation) Mendel made a
second discovery which also has far-reaching consequences. The following
case illustrates Mendel's second law.

If a pea that is yellow and round is crossed to one that is green and
wrinkled (fig. 21), all of the offspring are yellow and round. Inbred,
these give 9 yellow round, 3 green round, 3 yellow wrinkled, 1 green
wrinkled. All the yellows taken together are to the green as 3:1. All the
round taken together are to the wrinkled as three to one; but some of the
yellows are now wrinkled and some of the green are now round. There has
been a recombination of characters, while at the same time the results, for
each pair of characters taken separately, are in accord with Mendel's Law
of Segregation, (fig. 22). The second law of Mendel may be called the law
of independent assortment of different character pairs.

[Illustration: FIG. 21. Cross between yellow-round and green-wrinkled peas,
giving the 9: 3: 3: 1 ratio in F_2.]

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.

[Illustration: 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 on