FERTILITY IN ANIMALS 



555 



of certain Mendelian factors upon fertility cannot, therefore, well be 

 denied. (See Fig. 216.) 



The Chromosomes and Fertility. Bridges has demonstrated for 

 Drosophila that males of the chromosome constitution XO, instead of 

 the normal XY, are totally sterile. Here a specific chromosome differ- 

 ence, the absence of the F-chromosome from the hereditary 

 mechanism, leads definitely to complete sterility. Not many other 

 cases are known among animals of sterility dependent upon abnormal 

 chromosome constitution, but Bridges reports several known cases of 



FIG. 216.- 



- Drosophila mutation which exhibits a high degree of sterility, a, Normal wing; 

 6 and c, fused wings. (After Morgan and Bridges.) 



aberrant hereditary behavior which may be dependent upon irregular 

 chromosome distribution and content. 



Sterility in Other Animals. In some other animals there are cases of 

 sterility which suggest strongly the effect of definite Mendelian factors. 

 Thus several writers have commented upon the sterility of tortoise- 

 shell male cats, and apparently orange males are also, sometimes at 

 least, sterile. The reason for this particular case has not yet been 

 established definitely by breeding tests, and there is apparently some pos- 

 sibility that irregular chromosome distribution may account for it. 



An instance from practical breeding history which appears to belong 

 to this category is that of barrenness in Bates's famous Duchess family 

 of Shorthorns. This family was noted for superior individual excellence, 

 consequently breeders, naturally desirous of maintaining this excellence, 

 followed a practice of close breeding within the family, an example of 



