20 Sex-Limited Jnheritance in Cats 



schemes are available. Certain points of importance, however, may be 

 considered. It has been suggested by Little^ that both the black and 

 yellow factors are sex-limited in the male Cat, and this suggestion is in 

 harmony with the fact that a female heterozygous for yellow is tortoise- 

 shell while the heterozygous male is yellow. For if the factor for black 

 were not sex-limited in the male, one would expect the male heterozygous 

 for yellow to be tortoiseshell as in the case of the female. If all males 

 containing the yellow factor were yellow, and if there were no exceptions 

 to the rule that the yellow male transmits yellow only to his daughters, 

 the case of the Cat would fall quite simply into the scheme which I 

 recently suggested for characters which appear to be dominant in one 

 sex and recessive in the other-. This suggestion was that one sex 

 (in the Cat and in Man the female) is normally homozygous for a 

 factor for " normality " (NN), for which the normal male is heterozygous 

 (Nn), and that N is constantly coupled with a sex-determining factor 

 for which the female is homozygous, the male heterozygous. In the 

 Cat the factor for normality would be the black determiner, which may 

 therefore be written B instead of N ; then if X is used for the sex- 

 factor, adopting the notation commonly used in America, a black male 

 would be BX.bx, a black female BX.BX. If then the yellow coloui- be 

 caused by the loss of the factor B, the yellow female would be bB.bX, 

 the tortoiseshell female BX.bX, and the yellow male bX.bx. 



This scheme, however, has the obvious defect that it represents the 

 black male and the tortoiseshell female as of the same factorial composi- 

 tion as far as B and b are concerned (Bb in each case). An improvement 

 might be to represent the yellow flictor as a modification of B rather 

 than its absence; if the modified B which gives rise to yellow instead of 

 black be called Y, the yellow female would be YX.YX, the tortoise- 

 shell female BX.YX, and the yellow male YX.bx. Since Y is normally 

 coujjled with X, a yellow male would then normally transmit Y only 

 to his daughters, and since when he is mated with a black they would 

 receive B from their mother, they would be tortoiseshell, thus : — 



yellow male YX . bx x BX . BX black female 

 gametes bx, YX BX, BX 



black male bx . BX YX . BX tortoiseshell female. 



' Scienc; May 17, 1912, 2 Joiinml of Genetics, i. 1911, p. 377. 



