66 HEREDITY 



A more remarkable case has lately been discovered 

 by the American experimenter Castle in connection with 

 yellow mice. It has long been suspected that there was 

 something remarkable about them. In the first place, 

 they never breed true. They are somewhat delicate 

 as compared with ordinary mice, they are frequently 

 sterile, and many show a curious tendency to become 

 excessively fat. 



If yellow mice are mated with others of the ordinary 

 wild grey colour, the ratio obtained is about 50 per 

 cent, yellow, 50 per cent, non-yellow. This suggests 

 that the yellows are all hybrids, yellow being dominant 

 to grey. When a large number of yellows were inter- 

 bred by Castle the numbers 800 yellow, 435 non-yellow 

 were obtained, which, considering that the death-rate 

 is usually higher among the yellows, indicates the ratio 

 2 yellow : 1 non-yellow. This ratio is explained by 

 supposing that the pure yellow type, which ought to 

 form one-fourth of the whole, are incapable of sur- 

 viving ; and the supposition is confirmed by the fact 

 that matings of two yellows usually give smaller litters 

 than matings of yellow with non-yellow. It is im- 

 possible at present to say why a pure yellow mouse 

 should be an impossibility, for hair pigment in animals 

 has never been considered indispensable, in the way 

 that chlorophyll or leaf -green is to plants. The reason 

 then remains obscure, but the inheritance of the char- 

 acter itself is explained. 



A number of somewhat puzzling complications are 

 grouped as the effects of coupling and repulsion between 

 different factors. There is often a distinct tendency for j 

 characters which appear to have nothing to do with each 

 other, to be inherited together or to repel each other in 

 such a manner that a given two rarely occur in one in- 

 dividual. Thus, for example, the " hooded " form of the 

 standard petal in sweet-peas seems to be strongly coupled 

 with certain colours, and occurs only rarely in others. 

 This subject, however, has not been fully investigated. 



In other cases there appears to be some selective 

 mating between the reproductive cells. A sperm seems 



