HEREDITY OF HAIR-LENGTH IN GUINEA-PIGS. 1 3 



5. GAMETES OF CROSS-BREDS OFTEN IMPURE. 



From the foregoing observations it is clear that, while the long-haired 

 and short-haired conditions are sharply alternative to each other in heredity, 

 the gametes formed by cross-breds are not in all cases pure. Frequently 

 they consist of a blend or a mixture of the two alternative conditions, con- 

 stituting in effect a new condition intermediate between the other two. A 

 study of other characters alternative in heredity yields results somewhat 

 similar. 



Albinism is, in heredity, the most sharply alternative of characters, yet 

 cross-breeding between albino and pigmented guinea-pigs may modify the 

 character both of the albino race and of the pigmented one. This modifi- 

 cation may take on a variety of forms, as has elsewhere been pointed out 

 (Castle, :05). It may result in the production of mosaics (pigmented 

 animals spotted with white), or of albinos with a modified peripheral 

 pigmentation, or of albinos visibly like their ancestors but transmitting a 

 different set of latent characters. Again, the rough or rosetted coat of certain 

 races of guinea-pigs is sharply alternative to smooth coat, yet cross-breeding 

 of rough with smooth races may induce curious modifications of the rough 

 character or produce smooth individuals bearing the merest trace of the 

 rough character. 



All these facts are in harmony with the hypothesis, for which there is 

 strong evidence on the cytological side, that each separately heritable char- 

 acter is represented by a different structural element in the germ (egg or 

 spermatozoon). In fertilization the paternal and maternal representatives 

 of a character become more or less closely united, this union persisting 

 through all subsequent cell-generations until the new individual forms its 

 sexual elements. At that time the paternal and maternal representatives of 

 a character separate from each other and pass into different cells. 



But the paternal and maternal representatives of a character may in 

 the meantime have exercised on each other a considerable influence. In 

 the case of some characters, as ear-length in rabbits (Castle, :0s''), they com- 

 pletely blend and intermingle, so that a new character is produced strictly 

 intermediate between the conditions found in the respective parents. 



In other cases the modification may be slight, as if the paternal and 

 maternal representatives of a character had been scarcely more than approx- 

 imated. Sometimes in cases of alternative inheritance no influence of the 

 cross is observable in certain of the " extracted " individuals, but if any con- 

 siderable number of individuals is examined, others will be found in which 

 the cross-breeding manifests its influence. From this we conclude that 

 gametic purity is not absolute, even in sharply alternative inheritance. 



