280 Colour Inheritance in Cats 



(3) In crosses between two yellow cats, although only yellow young 

 are expected, two aberrant results have been noted. * 



(a) A mating of this type has produced tortoise-shell females 

 besides yellows of both sexes. (Doncaster, 1913.) 



(b) A mating of this type has produced tortoise-shell females 

 and black males besides yellows of both sexes. (Whiting, 1918.) 



(4) There is no record of two black cats crossed together having 

 given yellow or tortoise-shell young. 



(5) Tortoise-shell males are produced much more rarely than any 

 of the aberrant classes recorded under headings 1, 2, and 3 above. (Don- 

 caster, 1913; Wright, 1918.) 



(6) Such tortoise-shell males are usually sterile. (Cutler and Don- 

 caster, 1915.) 



(7) If they are not sterile they apparently do not give tortoise-shell 

 sons, but breed as yellows. (Doncaster, 1913.) 



In considering these facts, investigators have usually tried to explain 

 all of them by a single hypothesis. (Doncaster, 1913; Whiting, 1918.) 

 This has proved to be difficult and unsatisfactory. (Ibsen, 1916 ; Wright 

 1918.) 



It is believed that the experimental evidence favours the existence 

 of two genetically independent agents at work in the production of these 

 aberrances, for 



(a) The appearance of the unexpected individuals noted under 

 headings 1, 2, and 3 above, is relatively frequent, and produces regular 

 results involving neither sterility nor the formation of new colour types. 

 {b) On the other hand, the occurrence of tortoise-shell males is 

 very infrequent, not regular, and is in a majority of cases intimately 

 connected with sterility. 



Such being the case, an effort will be made to explain the appearance 

 of the unexpected individuals noted under headings 1, 2, and 3 by one 

 hypothesis and the occurrence of tortoise-shell males by a different one. 



III. The relation betv^een yellov^ and black. 



One of the first points to be established is the nature of the genetic 

 relation between yellow^ coat colour and black coat colour. 



In this connection Ibsen, 1916, and Wright, 1918, believe black or 

 extension of black pigment to the coat, to be epistatic to yellow or the 

 restriction of black pigment from the coat. Doncaster, 1913, and 



