288 Colour Inheritance in Cats 



If now such a peculiar yellow, YBX96, is mated with any female showing 

 primary non-disjunction — an animal which might well prove to be a 

 fertile tortoise-shell male would be produced. Thus : 



Here the assumption is made that an animal formed from the combination 

 of gametes, YBX 6 and -, may he somatically a tortoise-shell, and that the 

 6 chromosome which is brought into the zygote by an X-bearing gamete 

 does not in all cases exert its full influence until gametogenesis. The 

 FBX^-male would then be supposed to develop somatically just as does 

 the FSX-animal, but upon gametogenesis the 6 chromosome of the YBX6- 

 male is able to prevent the sterility which exists in its absence. This 

 seems quite possible, for it appears that in Drosophila the 6 chromosome 

 is not needed for the development of the normal male somatic characters, 

 but that it is necessary, however, for successful gametogenesis in the 

 male. 



A fertile tortoise-shell male would, when he formed gametes, behave 

 exactly like a normal yellow male. That is to say, although he was 

 himself the product of a combination of Xd and - gametes, he would in 

 gametogenesis form only X and 6 gametes, just as would a normal male. 

 This has been the breeding behaviour of the one recorded certainly fertile 

 tortoise-shell male (see Doncaster, 1913) which acted in crosses with 

 tortoise-shell females apparently exactly as a yellow male would have 

 done. 



It wdll be seen that the above hypothesis, although somewhat com- 

 plicated, is nevertheless in accordance with experimental facts and 

 accounts for sterile and fertile types of tortoise-shell males ; it explains 

 their infrequency of appearance, and possibly their failure to transmit 

 their own colour pattern to their descendants ; it is supported by the luork 

 of Bridges with Drosophila — the most completely investigated form showing 

 a similar type of sex-linkage ; it is further capable of experimental tests. 



