FACTOR RELATIONS IN QUANTITATIVE INHERITANCE 177 



is involved, and the bush-Cupid correspondingly combines the charac- 

 teristics of the bush and Cupid sweet peas. Like the Cupid sweet pea 

 it is a very dwarf form, but it is erect and much branched like the bush 

 sweet pea. In this particular instance, therefore, the combination of 

 recessive factors results in a combination of the characters which these 

 factors ordinarily determine. The segregation gives four distinct types 

 in FZ which are easily distinguishable one from another at maturity, and 

 which behave in characteristic manners in the following F 3 generation 

 as is indicated by the accompanying checkerboard. The case, while 

 more complex than those in which only one factor difference is responsible 

 for the character differences, is still simple and readily analyzed by pedi- 

 gree culture methods. Figs. 123 and 124 illustrate these forms of sweet 

 peas. 



In Drosophila there is at least one size difference dependent upon a 

 simple factor difference. This factor belongs in the third group and 

 bears the same relations to the other factors of the genetic system as are 

 displayed by any other locus. The factor dwarf and the corresponding 

 character are in every respect strictly analogous to true qualitative factors. 

 In general the facts of heredity with regard to the simple size differences 

 which we have noted are in agreement with the interpretation that they 

 have arisen by some mutation in a single locus in the normal form and 

 therefore naturally enough display simple monohybrid inheritance. 

 In the case of Lima beans the origin of the bush type by mutation was 

 definitely observed and similarly the Cupid sweet pea was discovered 

 among a population of tall forms. There is no reason, therefore, for 

 advancing any different explanations than those here given for size differ- 

 ences of this type. 



There are other size differences which are not particularly different 

 from those above noted except that they are in the opposite direction. 

 A case in point is the mutation in tobacco discovered by East and Hayes. 

 This mutation is one in which the habit of growth has been changed from 

 the ordinary determinate type in which an inflorescence terminates the 

 axis at a relatively invariable stage of elongation to one in which the axis 

 continues growing and producing leaves for a considerably longer period 

 of time. This giant type of tobacco is a recessive form, and like the size 

 differences above noted represents a single factor difference from the 

 normal type (see Fig. 150). In Drosophila there is a factor for giant size 

 in the third group which gives rise to a form several times larger than the 

 normal type, and like the factor for dwarf, which is in the same chromo- 

 some, there is no difference between it and its relation to other factors 

 and the relation of these factors to each other. These facts like those 

 which have been discussed above merely serve to emphasize the fact that 

 size differences, some of them extremely wide, may depend upon simple 

 12 



