Let me illustrate by an example in which the hybrid standing between two types overlaps them both. There are two mutant races in our cultures of the fruit fly Drosophila that have dark body color, one called sooty, another which is even blacker, called ebony (fig. 20). Sooty crossed to ebony gives offspring that are intermediate in color. Some of them are so much like sooty that they cannot be distinguished from sooty. At the other extreme some of the hybrids are as dark as the lightest of the ebony flies. If these hybrids are inbred there is a continuous series of individuals, sooties, intermediates and ebonies. Which color here shall 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.
Fig. 20. Cross between two allelomorphic races of Drosophila, sooty and ebony, that give a completely graded series in F2.
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.
