394 EVOLUTION, GENETICS, AND EUGENICS 



Mendel had no knowledge of chromosomes or of the chromosomal 

 mechanism of maturation, which now seems to be the machine respon- 

 sible for the regularities seen in the various Mendelian ratios and for 

 segregation in general. It is remarkable, however, that Mendel fore- 

 saw a mechanism within the genetic apparatus of plants that coin- 

 cides in principle with that subsequently discovered. Among the great 

 discoveries that have resulted from the use of Mendelian methods 

 and procedures are the factor hypothesis, the chromosome theory of 

 heredity and of sex determination, linkage and crossing-over, and the * 

 finer details of the heredity machine. 



THE FACTOR HYPOTHESIS 



The factor hypothesis is a neo-Mendelian concept, by which we 

 mean that it was unknown to Mendel. According to Mendel, each 

 unit character was determined by a single determiner in the germ cell. 

 Also, the determiners of a pair of allelomorphs were both positive in 

 character and opposed to each other in their effects. The presence- 

 and-absence hypothesis, according to which a recessive gene differs from 

 a dominant gene merely in lacking something possessed by the domi- 

 nant gene, opened the way toward a much more satisfactory under- 

 standing of the ways in which germinal determiners (genes or factors) 

 influence characters than was possible under the view of Mendel. 

 When once we learn that a single character may depend upon the mter- 

 action of two or more independently inherited and segregating factors 

 or genes, it becomes possible to understand all sorts of puzzling and 

 apparently non-Mendelian ratios. The adoption of the factor hypoth- 

 esis has justified itself over and over again, for it has been the instru- 

 ment 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 fundamentally different. Thus there is now no fur- 

 ther 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 ("cumulative" or "duplicate factors" of some au- 

 thors) 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 con- 

 nection with the discovery 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 



