EXAMPLES OF MULTIPLE FACTORS. 13 



FORM DIFFERENCES. 



From quantitative differences in color one is led to quantitative dif- 

 ferences in shapes and forms. In crossing round and long varieties of 

 Brassica roots Kajanus (1912) found an intermediate F x and an F 2 in 

 which occurred distinct longs, various intermediates, and rounds in the 

 proportions 1 : 14:1. To explain this and similar phenomena Kajanus 

 assumes that there are two factors that lengthen the root (p. 225) : 



"Die jedes fur sich bei doppelten Vorhandensein, und vereint bei einfachen 

 Vorkommen eine zwischen ausgepragt langer und ausgepragt kurzer Form 

 genaue Mittelstellung bedingen und bei sonstigen Kombination andere Formen 

 zwischen lang und kurz hervorrufen." 



Kajanus (1911) describes more complicated conditions in crosses with 

 Beta. Nine types, fairly clearly separable, were recognized, ranging 

 from flattened round to very long and slender. Thirty-two crosses 

 were made and large numbers were obtained in F 2 . The results show 

 a fair degree of similarity with the following working hypothesis : There 

 are two factors that influence length and two that influence roundness. 

 Different combinations of these four factors and their absences account 

 for the different types. 



Emerson (1910) reports crosses between varieties of summer squashes 

 and of gourds (Cucurbita pepo) which differ in shape of fruit. By a 

 comparison of the coefficients of variability of Fi and F 2 , it was found 

 that there was a marked increase in variability in the second generation. 



This increase in variability is strikingly similar to the new grades of 

 yellow in the theoretical case of yellow peas, to the grades of color in 

 cereal crosses, and to the variety of forms found in the second genera- 

 tions of crosses of Brassica and Beta. Although Emerson makes no 

 attempt to analyze the various grades in his crosses and no guess as 

 to the number of factors involved, there seems little doubt but that 

 (as he concludes) evidences of segregation are clear. 



These two cases of shape differences involved incomplete dominance, 

 or its absence. Shull (19116) described a case in Bursa bursa-pastoris, 

 in which two distinct form types are supposed to differ by two inde- 

 pendent genes. The seed capsule in Bursa bursa-pastoris is flat and 

 triangular; that of a recent mutant, heegeri, is oval; while in section it 

 is round. The pastoris form is dominant ; the heegeri form appears only 

 in the absence of both the factors assumed to cause the pastoris type. 

 This supposition was based on the following ratios : F 2 = 21 .9 : 1 (includ- 

 ing 2,540 plants); F 3 = 4.67:1 and 22.2:1. These were believed to 

 represent 15:1, 3:1, and 15:1 respectively. The uniformity of the 

 deviation from the expected ratios is believed not to be due to chance 

 or to any failure of segregation, but to some modifying influence that 

 perhaps prevented the development of certain combinations, as is sup- 

 posed to be the case by Castle and Little (1910) in yellow mice and 

 as has actually been observed in Antirrhinum by Baur (1910a). 



