282 Colour Inheritance in Cats 



IV. An attempt to explain the appearance of unexpected indi- 

 viduals OF NORMAL COLOUR TYPES. (Headings 1, 2, and 3, 

 Section ii, above.) 



It is tacitly assumed by all investigators that at some time or times 

 in the past, there must have been a genetic change, ridding certain 

 gametes of the epistatic colour factor, whether it be the Y of Whiting, 

 the E of Wright, or the T of Ibsen. Had this not been the case neither 

 the hypostatic form nor the tortoise-shell heterozygote could have 

 appeared. 



We may, then, for the sake of argument accept the set of symbols 

 given above, and assume that tbe change from Y lo y must have occurred. 

 There is no experimental evidence to show how recently or how fre- 

 quently this change may have taken place, but if we assume that it is 

 still taking place in a portion of the gametes of certain individuals — 

 which seems entirely probable — all the results obtained under headings 

 1, 2, and 3, may be accounted for. Such a change from an epistatic to 

 a hypostatic condition would be directly comparable to the appearance 

 of the recessive pink-eyed mutation in a stock of dilute brown mice 

 recorded by the writer in 1916. 



Animals in whose gametes this mutative process was occurring de 

 novo would show no trace of it in their own somatic characteristics, but 

 would, upon breeding, give results in agreement with the actual aberrant 

 classes obtained. 



We should thus expect that an occasional yellow female would 

 form gametes yBX in addition to those containing YBX which she 

 normally produces. Similarly, certain yellow males would be found which 

 showed by their progeny that they were forming among their X gametes 

 some which were of the constitution yBX instead of the normal YBX 

 type. 



Yellow males of this unusual kind would, when crossed with black 

 females, give among their progeny a certain number of black females, in 

 number depending upon the frequency with which the unusual yBX 

 sperm was formed. This fact would explain the aberrances listed above 

 under Section ii. Heading 1. 



Similarly, such unusual yellow males would, when mated to normal 

 tortoise-shell females, give rise to a certain number of black females in 

 addition to the other classes normally expected. This would cover 

 category two of exceptions mentioned above (Section ii). 



Finally, a yellow forming yBX gametes, when crossed with a normal 

 yellow or with one of its own type, would give rise to unexpected black 



