No. 578] MULTIPLE ALLELOMORPHS 



were certainly not blacks, since they had light bellies, 

 but which died before attaining the age at which gray 

 can be distinguished from black- and-tan. It is certain 

 that among the 109 young produced by the 8 animals of 

 type (2) not a single one was black. 



But if black-and-tan is not an actual allelomorph of 

 gray, black young as well as black-and-tans should have 

 been produced in the foregoing case. For if black-and- 

 tan is not allelomorphic with gray, or is due to an inde- 

 pendent inhibitor of gray, then an F t gray should produce 

 gametes of four sorts, rather than as indicated of two 

 sorts ; i. e., gametes should arise which transmit both gray 

 and black-and-tan, and others which transmit neither gray 

 nor black-and-tan. The former sort possibly might not 

 be capable of immediate detection in the back-cross with 

 black, but the latter should be readily discovered since 

 they would necessarily produce black young (neither gray 

 nor black-and-tan). The total absence of black young 

 from the litters produced by type (2) matings therefore 

 indicates strongly that gray and black-and-tan are allelo- 

 morphs of each other. 



(b) An alternative view, however, deserves considera- 

 tion. If gray and black-and-tan are not actual allelo- 

 morphs, it is conceivable that they may each be closely 

 "coupled" with a common structure in the germ cells and 

 so behave as allelomorphs under ordinary circumstances, 

 though not being such in reality. Or, what would give 

 the same practical result, gray and black-and-tan might 

 be supposed to contain the same agouti factor, but this 

 might be considered in one as closely coupled with a modi- 

 fying factor which made its action different. Neither 

 form of this hypothesis is capable of proof or disproof, 

 for which reason alone the hypothesis is unimportant, but 

 its probability grows less the larger the number of records 

 obtained which show no breaking of the supposed coup- 

 ling. Our cases are not as yet numerous enough to throw 

 much light on this question, but so many cases have 

 already been discovered in which characters assume three 

 or more mutually allelomorphic conditions and in which 



