THE TORTOISESHELL TOMCAT — A SUGGESTION. 



By L. DONCASTER, F.R.S. 

 Professor of Zoology, Liverpool University. 



In the Journal of Genetics, viii. 1919, p. 279, C. C. Little puts forward 

 a new hypothesis to account for the origin of the tortoiseshell tomcat. 

 He suggests that the sterile tortoiseshell male is comparable with the 

 XO males produced in Drosophila by the fertilization of a non- 

 disjunctional ovum bearing no sex-chromosome by an X-bearing sper- 

 matozoon. Such males in Drosophila are sterile, and not infrequently 

 show mosaic characters. Little therefore suggests that the sterile 

 tortoiseshell male is of similar origin, is sterile from the same cause, 

 and is tortoiseshell instead of yellow owing to a tendency to mosaic 

 distribution of the yellow factor. Fertile tortoiseshell males are ex- 

 plained on the assumption that by secondary non -disjunction the Xand 

 Y (X and 6) chromosomes come into the zygote from the same parent, 

 and it is suggested that under these circumstances the " " chromosome 

 does not have its nonnal effect, and that what would otherwise have 

 been a yellow male becomes a tortoiseshell. For this second assumption 

 there is little or no real evidence, and its author himself seems somewhat 

 diffident about it. The comparison of the sterile tortoiseshell male with 

 the sterile XO type of male in Drosophila is at first sight more attractive. 

 The comparison, however, really rests only on the fact of sterility, for the 

 tendency to a mosaic distribution of factors does not seem to be compar- 

 able in the two cases. In Drosophila, as Morgan, Bridges and Sturtevant 

 show in their recent monograph on Drosophila (7), the mosaic distri- 

 bution of factors is almost certainly due to abnormal distribution 

 of the X-chromosome in the segmentation of the egg, and the flies 

 are almost always mosaics of sex-characters (gynandromorphs) no less 

 than of inherited factors. The inherited factors accompany the ab- 

 normal distribution of the sex-factors, so that dififerent parts of the 

 fly show not only different inherited characters but also dififerent sexual 



Journ. of Gen. ix 22 



