The second principle of inheritance discovered through the pea experiments is known as the Law of Independent Assortment. It states that the behavior of each homologous pair of genes in a hybrid, and presumably of each homologous pair of genes in a pure stock also, is independent of the behavior of all other pairs of genes.

The experiment which led to the enunciation of this law was a cross between a pea plant having round and yellow seeds with one having wrinkled and green seeds. The hybrid individuals were round and yellow, for these were the dominant characteristics. But the progeny of the hybrids were not solely of the grandparental types. Instead, the two contrasting pairs of characters appeared in all of the combinations possible. There were round and yellow peas, round and green peas, wrinkled and yellow peas, and wrinkled and green peas; and the proportions of these types invariably approached the ratio 9:3:3:1. Plainly such a result could only be obtained if the germ cells of the hybrids had received one member of each pair of genes by independent segregation. This would mean, if the two genetic differences be represented by Aa and Bb, that the four types of germ cells AB, Ab, aB, and ab were formed in equal quantities. If one multiplies egg cells AB + Ab + aB + ab by sperm cells AB + Ab + aB + ab, having regard for the phenomenon of dominance, plants duplex in genetic constitution are obtained in the 9:3:3:1 ratio. This was the situation actually found.

Numerous experiments modeled after those of Mendel were carried out during the early years of the twentieth century; and gradually more and more complex cases of inheritance yielded to factorial analysis. All possible modifications of the fundamental di-hybrid ratio (also tri-hybrid and higher ratios) of 9:3:3:1 were discovered. These modifications were due to the fact that certain types of genes produced results which made two or more of the members of this ratio resemble each other superficially, though their breeding powers were quite different. Thus hybrids produced populations which could be divided into distinct groups in such ratios as 13:3, 9:7, and 9:3:4; yet continued breeding tests proved that each was simply a modified 9:3:3:1 ratio.

Perhaps the most important result of this early work was the replacement of the old notion that like produces like by the idea that each gene produces genes like itself. Exact analysis of pedigree culture data had shown that individuals which could not be distinguished from each other by any ordinary test produced progenies which did not resemble each other. Part of this interesting situation was cleared up by the recognition of the important role played by dominance, since AABB individuals would look like AaBB, AABb, and AaBb individuals, though each type would behave differently in reproduction. But this was not the whole story. It was found that genes AA could be the determinative factors in the production of characters that would hide the characters produced by genes BB. In such cases individuals containing the genes AA (or Aa) would be alike in appearance whether they contained genes BB (or Bb) or not. Moreover, it was discovered that sometimes two genes, let us say AA and BB, had to be present together for the production of a detectable character. Thus a white sweet pea AAbb, crossed with another white sweet pea aaBB, produces a purple hybrid AaBb, since both the A gene and the B gene are required for the manufacture of the purple pigment. This discovery, by the way, cleared up a hoary old biological puzzle - namely, why certain crosses show reversion to the type characteristic of some bygone ancestor.

The numerous minor discoveries made during this period need not concern us here, since the objective of the first part of this essay is the groundwork of present-day genetic philosophy. It is necessary, however, to call attention to the general concept of gene behavior which slowly took form in the minds of genetic investigators. From the first it did not seem probable that even such an apparently unimportant gene as, for example, the one which differentiates brown eyes from blue in the human race, could have only a single function. Were this true, an organism would be merely a genetic mosaic, a piece of animated tiling. And this proved not to be the case. Genes are units in inheritance, but are not units in development. Development is a coordinated affair which one may think of as the progressive unfolding of the organism as a whole. In this unfolding a single gene has manifold duties, and numerous genes contribute their quotas toward the fulfillment of a seemingly simple task. For instance, one must assume that the mammalian eye is the result of the combined activity of hundreds of genes; one must also assume that each of these genes, though primarily concerned with eye development, may have minor effects on many other organs.

It took only five or six years from the beginning of the genetic renaissance, which came with the rediscovery in 1900 of Mendel's thirty-five-year-old paper, for biologists to realize that the Mendelian type of inheritance derived from some fundamental mechanism. This conclusion was justified because Mendelian inheritance had been found in various orders of flowering plants and, among animals, in mollusks, insects, fishes, birds, and mammals. Man himself was no exception. Carefully collected genealogies had already shown that a dozen or more contrasting conditions such as sound mentality and feeble-mindedness, ordinary stature and dwarfness, and normal vision and color-blindness were due to differences in a single gene.

In spite of the facts available at this time, the criticism was often raised that this interpretation could not serve for all forms of inheritance, even in a given species, because quantitative characters - that is to say, size characters - could not possibly be inherited in this manner. These objections stimulated work upon size inheritance; and shortly the Theory of Multiple Factors, a theory which brought the inheritance of all classes of characters under one interpretation, was announced. It consisted merely in the assumption that genes Aa, Bb, Cc, etc., can exert cumulative effects directed either toward the development of the whole body or toward that of a single organ. Because of the complexity of such inheritance, and more especially because of the extraordinary effects which slight differences in environmental factors have upon size characters, this theory was difficult to prove. But by analyzing statistically the results from ordinary Mendelian inheritance, and then using the statistical procedure to analyze experimental data, satisfactory proofs were offered.