284 Colour Inheritance in Cats 



factors determining the relative degree of black and yellow pigmentation. 

 They may be separately considered as follows : 



(a) Doncaster's hypothesis of a break in sex-linkage : this hypothesis, 

 which in a modified form is a basis for Ibsen's later explanation of the 

 appearance of unusual colour types, involves, if it is to explain the ex- 

 ceptional black females, the existence of "crossing over" in the male 

 between the X and the 6 chromosomes. Such crossing over has not, in 

 so far as I am aware, been demonstrated in any forms in which the male 

 is XO in formula as in cats. It further would suppose that, as tortoise- 

 shell males were formed by the same process, they would be expected to 

 occur with as great frequency as the exceptional black females. It 

 further leaves entirely unexplained the appearance of blacks or tortoise- 

 shells from a cross between yellow animals. These objections seem to 

 be of sufficient weight to throw the chances against Doncaster's or Ibsen's 

 hypotheses. 



(6) Whiting's hypothesis of modifying factors which at one end of 

 the series would serve to make tortoise-shell animals yellow, and at the 

 other end of the series make them black, remains as a possibility though 

 seriously invalidated by certain points as follows : 



(1) There should be records of black females (geno typically 

 tortoise-shell) which if crossed with other blacks should give yellow 

 males and tortoise-shell females, or if crossed with yellow males should 

 give unexpected yellow females. Neither of these results has been 

 recorded. 



(2) Doncaster, 1913, reports that the three tortoise-shell females 

 produced from a single cross between two yellows were " blue with a 

 cream patch " thus showing that they were near the black end of Whiting's 

 modifier series. Inasmuch as under his hypothesis one of their parents 

 must have been at the opposite or yellow end of the series, it is difficult 

 to explain ho w and why many of its progeny should show the condition 

 characterising nearly the other end of a graded series. 



(B) The occurrence of these young in a single mating makes it 

 seem likely that the particular animal was forming ordinary yBX gametes 

 in a considerable number. 



(4) The tortoise-shell young produced by dilute yellow female 

 ^ 23 already referred to, before she was sent to Dr Whiting, were normal 

 tortoise-shell in colour ; if anything, more nearly on the black end of the 

 graded series, than on the yellow. This case serves to support that 



