L. DONCASTER 21 



If now, exceptionally, Y became dissociated from X in the gametes 

 of the yellow male, and were thus transmitted to a son instead of a 

 daughter, the result would be 



yellow male YX . bx x BX . BX black female 

 gametes Yx, bX BX, BX 



tortoiseshell male Yx . BX b y- . BX black female. 



The female offspring would in this case be black, but heterozygous 

 for B instead of homozygous, and the male, since he contains both Y and 

 B, would be tortoiseshell. 



I have one record of a tortoiseshell male produced by tortoiseshell 

 female by black male. The kitten died when only a month old, but 

 the breeder is confident that it was a male. If, as suggested, the 

 tortoiseshell male has the constitution YBXx, this could only arise in 

 this instance by the tortoiseshell mother transmitting YX, the black 

 father Bx ; the tortoiseshell male would then arise by the exceptional 

 association of B with x in the spermatozoa, instead of B with X\ 



The scheme outlined above is satisfactory up to this point, but it 

 has two disadvantages. It involves the assumption that the occasional 

 black females produced from yellow sires are of different constitution 

 from normal blacks (BX.bX instead of BX.BX), for which there is no 

 direct evidence. The point could be tested by mating such a black 

 female with an orange male ; if the female were heterozygous for B, it 

 would then give not only tortoiseshell females YX.BX, but also yellow 

 females of constitution YX . bX. This experiment has never, as far 

 as I know, been made. The second objection is more serious. The 



1 There is an additional piece of evidence supporting the hypothesis that the factor B 

 is sex-limited. I have a record of tabbies being produced from yellow <f x black ? . Tabby 

 is known to be epistatic to black, but presumably can only appear in the presence of the 

 factor B. The yellow male which transmits the tabby factor does so to both sexes, the 

 male kittens which receive the tabby factor being brown tabbies, the females (only one 

 recorded) " tabby- tortoiseshells." It is clear therefore that the tabby factor is not sex- 

 limited. If, however, the black factor were present in the orange male, it is probable 

 that the cat would be tabby or tabby-and-yellow instead of yellow. If, therefore, a yellow 

 male can in some cases transmit the tabby factor, additional evidence is provided for the 

 assumption that the black factor is not present. Tabby tortoiseshell males, though very 

 rare, do ejtist, and are presumably tortoiseshell males which bear the tabby factor in 

 addition. One was exhibited at the Crystal Palace Show in 1912. 



