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 YO, instead of 
the normal XY, are totally sterile. Here a specific chromosome differ- 
ence, the absence of the Y-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 
Fie. 216.—Drosophila mutation which exhibits a high degree of sterility. a, Normal wing; 
b 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 
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