vn] Yellow Mice 119 



impossible to bring the whole series of observations into 

 one consistent scheme. For first, Cuenot (Z"]) has shown 

 that in the mouse yellow is dominant to black, instead of 

 being recessive to it as in the rabbit. The dominance is so 

 far incomplete that yellows heterozygous with black have 

 often — perhaps always — more or less black pigment in their 

 hairs, but in general appearance they are obviously yellow. 



Next according to Cu^not's experience none of the 

 yellow individuals in F^ from a cross of yellow x black or 

 yellow X agouti are pure to yellow '^. All such extracted 

 yellows when bred together throw agoutis or blacks respec- 

 tively. This fact has led him to suggest that there may 

 be some incompatibility which prevents two yellow-bearing 

 gametes from uniting in fertilisation. These " impure " 

 yellows bred together gave 232 yellows and 86 agoutis, a 

 fairly close approximation to 3 : i. The expectation on 

 that ratio is 238*5 : 79*5. Cuenot, commenting on the 

 numbers produced, remarks that since pure yellows are not 

 formed, he would anticipate a diminution in the relative 

 numbers of yellows, and that such a diminution actually 

 occurs. But, as Mr Punnett has pointed out to me, there 

 seems to be no valid reason for expecting a departure from 

 the ratio 3:1; for the spermatozoa may be regarded as 

 unlimited in number. Of the "non-yellow" ova half would 

 be fertilised by "non-yellow" sperm, producing agoutis, the 

 other half being fertilised by "yellow" sperm and pro- 

 ducing yellows. The "yellow" ova would all, on Cuenot's 

 hypothesis, be fertilised by " non-yellow " sperm, and give 

 rise to yellows. The nett result would then be still 

 3 yellows : i agouti. 



Of 81 F^ yellows tested by Cuenot, all proved to be 

 heterozygous, so that the reality of the abnormal pheno- 

 menon must be regarded as established. Observations made 



* A fact of a different order, though perhaps having a bearing on the 

 problem, was observed by Hurst in crosses between Belgian Hares 

 (Rabbits) and Angoras. The Belgian is in colour an "agouti," or grey, 

 but it is much yellower than the wild rabbit of the warrens. Hurst found 

 that F^ from such Belgians and Angoras (bearing G) was of the wild or 

 warren type of grey, and that in JF^ no rabbit quite of the Belgian colour 

 ever came. The greys in F^ varied a little in the amount oi yellow, and it 

 is possible that, as in other cases in which a parental type fails to recur in 

 F^i the absence may be ascribed to the rarity of a particular combination. 



