316 EVOLUTION AND GENETICS 



to suppose that a character which has given rise to one mutation 

 is thereby prevented from giving rise to others. Multiple muta- 

 tions of a character, since they are allclomorphic to the original 

 character, would logically be allelomorphic to each other when 

 brought together in one individual. This explanation is in accord- 

 ance with all available data in the chromosome theory of descent. 

 It is adequate without further amplification, which may be found 

 if desired in the work cited above. 



Multiple Factors. Although the laws of heredity thus far 

 considered have to do with characters determined by single factors, 

 there are many different cases on record which must be explained 

 on the assumption that more than one factor is concerned in the 

 production of a single character. Extensive data have led to the 

 discovery of four kinds of multiple factors, known respectively as 

 duplicate, cumulative, complementary, and supplementary. 



Duplicate Factors. Shull discovered that the determination 

 of seed-capsule shape in shepherd's purse is accomplished by the 

 action of two pairs of factors. The seed capsules of this common 

 plant are usually triangular in outline but may be fusiform. 

 When plants of the two varieties are crossed, the Fi generation 

 has triangular capsules, and only one individual in sixteen in the 

 F2 generation reverts to the recessive spindle-shaped condition. 

 This is reminiscent of the dihybrid ratio, but an examination of 

 Figure 182 shows that the presence of a single dominant determiner 

 results in the appearance of the dominant character, and only the 

 homozygous recessive reveals its genotypic character. The be- 

 haviour of factors in this case is similar to that of all multiple 

 factors, but the appearance of the resulting characters differs in 

 this and the three following cases. 



More than two pairs of duplicate factors may govern a character ; 

 their distinctive quality is the similarity of their effect, no matter 

 how many of the dominant determiners are present. 



Cumulative Factors. Those factors which bring a character to 

 expression in the soma in proportion to the number of dominant 

 determiners present are called cumulative factors. The case of 

 Nilsson-Ehle's wheat is an old and excellent example. 



Nilsson-Ehle found that a race of wheat with red grains and 

 a race with white grains, when crossed produced an Fi hybrid with 

 intermediate pale red grains. In the F2 generation very few white 

 grains appeared, so that the color was obviously not due to a 



