IX] Unions betwee^t Gametes 163 



232 yellows and 86 agoutis, which is a near approach to 

 the normal 3 : i (238 '5 : 79*5). Cuenot comments on this 

 as a difficulty in the way of his view, saying that he would 

 have expected the ratio 2:1; but as Mr Punnett pointed 

 out to me, if all the ova bearing the yellow factor were 

 fertilised by agouti spermatozoa, the pumber of these being 

 indefinite, the chances of the non-yellow ova being fertilised 

 by a spermatozoon bearing yellow or non-yellow would 

 remain sensibly equal. Thus the ratio 3 yellow : i agouti 

 would result. 



Nevertheless the impression left on my mind by these 

 observations, and indeed by other strange phenomena which 

 yellows exhibit, is that the genetics of yellow mice are very 

 imperfectly investigated and that it is premature to formu- 

 late definite views as to their behaviour^. 



One of the peculiarities of yellow mice, well known 

 to fanciers, is their frequent tendency to excessive fatness. 

 Miss Durham, who has had considerable experience with 

 yellows, finds that this condition is not universal among 

 them, but shows itself in frequent individuals. She has 

 also found the genetic investigation of yellows very difficult 

 on account of the fact that they are often sterile, and the 

 suggestion is perhaps worth considering that this sterility 

 may be responsible for some of the complications, 



* Miss Durham has recently proved beyond all reasonable doubt that 

 the true account of this case is not that the two 'yellow' gametes are 

 incapable of uniting, but that zygotes so formed perish at some stage 

 before birth. For some physiological reason unknown, they are incapable 

 of living. From her own and other records the result of yellow x yellow 

 is now 151 1 yellows and 767 non-yellow, a close approach to 2 : i, which 

 gives the expectation 1518-6 : 759-3. The case, in fact, is analogous to 

 that of Baur's golden Antirrhinum (p. 253), in which the pure albinos are 

 missing, leaving a ratio of 2 golden : i green. (See Durham, Journ. 

 Genetics^ i. 1911, p. 167.) 



II — 2 



