276 EVOLUTION, GENETICS, AND EUGENICS 



It remains to be discovered to what extent each character is the result 

 of the interaction of all the genes. When once we learn that a single 

 character may depend upon the interaction of two or more inde- 

 pendently inherited and segregating factors or genes, it becomes pos- 

 sible to understand all sorts of puzzling and apparently non-Mendelian 

 ratios. The adoption of the factor hypothesis has justified itself over 

 and over again, for it has been the instrument that has led to a really 

 scientific genetics and has served to bring under one category all sorts 

 of hereditary phenomena that had formerly been considered funda- 

 mentally different. Thus, there is now no further need for the three 

 categories of heredity: alternative, blending, and particulate. All 

 three are now seen to be phases of alternative, or Mendelian, heredity. 

 Especially striking is the way in which the idea of multiple factors ("cu- 

 multative" or "duplicate factors" of some authors) has served to 

 rationalize and to bring into line with other Mendelian phenomena the 

 data about the inheritance of quantitative characters. Another 

 service of the factor hypothesis comes out in connection with the dis- 

 covery of lethal factors. There is a large number of genes or factors 

 whose presence in the homozygous condition (i.e., when a given factor 

 is present in both gametes that unite to form a zygote) leaves the indi- 

 vidual derived from such a zygote lacking in something essential for 

 life. All such individuals in any breeding experiment will fail to sur- 

 vive, and their absence will be noted when the ratios of the various 

 combinations are worked out. The failure of a certain expected com- 

 bination to appear in the F 2 generation is attributed to the presence of 

 a lethal factor in the stock. It can readily be proven that many of the 

 surviving individuals possess the lethal factor in a heterozygous condi- 

 tion, having one dose of the normal allelomorph along with the lethal 

 factor. These lethal factors can be identified and located as readily as 

 characters that actually appear. The subsidiary hypothesis of lethal 

 factors has had a far-reaching influence upon some of the most ad- 

 vanced phases of modern genetic practice. 



