INTRODUCTORY. II 



to imply that new characters have not appeared, or that those that 

 have appeared must be due to loss in the sense of absence of something 

 from the germ-plasm. 



In regard to the first point, several of the mutants are characterized 

 by what seem to be additions. For example, the eye-color sepia is 

 darker than the ordinary red. At least three new markings have been 

 added to the thorax. A speck has appeared at the base of the wing, etc. 

 These are recessive characters, it is true, but the character "streak," 

 which consists of a dark band added to the thorax, is a dominant. If 

 dominance is supposed to be a criterion as to "presence," then it should 

 be pointed out that among the mutants of Drosophila a number of 

 dominant types occur. But clearly we are not justified by these criteria 

 in inferring anything whatever in regard to the nature of the change 

 that takes place in the germ-plasm. Probably the only data which 

 give a basis for attempting to decide the nature of the change in the 

 germ-plasm are from cases where multiple allelomorphs are found. 

 Several such cases are known to us, and two of these are found in the 

 X chromosome group, namely, a quadruple system (white, eosin, cherry, 

 red), and a triple system (yellow, spot, gray). In such cases each 

 member acts as the allelomorph of any other member, and only two 

 can occur in any one female, and only one in any male. If the normal 

 allelomorph is thought of as the positive character, which one of the 

 mutants is due to its loss or to its absence? If each is produced by a 

 loss it must be a different loss that acts as an allelomorph to the other 

 loss. This is obviously absurd unless a different idea from the one 

 usually promulgated in regard to "absence" is held. 



MULTIPLE ALLELOMORPHS. 



It appears that Cuenot was the first to find a case (in mice) in which 

 the results could be explained on the basis that more than two 

 factors may stand in the relation of allelomorphs to each other. In 

 other words, a given factor may become the partner of more than one 

 other factor, although, in any one individual, no more than two factors 

 stand in this relation. While it appears that his evidence as published 

 was not demonstrative, and that, at the time he wrote, the possibility 

 of such results being due to very close linkage could not have been 

 appreciated as an alternative explanation, nevertheless it remains 

 that Cuenot was right in his interpretation of his results and that the 

 factors for yellow, gray, gray white-belly, and black in mice form a 

 system of quadruple allelomorphs. 



There are at least two such systems among the factors in the first 

 chromosome in Drosophila. The first of these includes the factor for 

 white eyes, that for eosin eyes, and that for cherry eyes, and of course 

 that allelomorph of these factors present in the wild fly and which 

 when present gives the red color. In this instance the normal allelo- 



