554 GENETICS IN RELATION TO AGRICULTURE 



Fertility as Related to Mendelian Factors. There is a considerable 

 body of evidence to show that some Mendelian factors exhibit residual 

 effects upon the fertility of individuals which bear them. This is perhaps 

 most clearly established for certain factors in Drosophila. Thus among 

 sex-linked factors Morgan has shown that those for the rudi- 

 mentary and the fused wing conditions are practically always associated 

 with sterility. In rudimentary flies the males are fully fertile, but the 

 females are usually completely sterile. Examinations of the ovaries of 

 rudimentary females demonstrate that the eggs do not develop normally, 

 but for the most part remain in a low stage of development. Similarly 

 the mutant fused is absolutely sterile in the female sex, but fertile in the 

 male. Stock must, therefore, be maintained by mating heterozygous 

 females to fused males. Here again examination of the ovaries has shown 

 reduction in the number of mature eggs normal for the wild type. 



Between this relatively complete sterility and the normal fertility of 

 the wild type there exist all possible gradations. In fact even in wild 

 type flies as Castle and his associates and others have abundantly shown 

 strains possessing different degrees of fertility exist. But mutant strains 

 often exhibit lessened vigor and fertility specifically attributable to the 

 residual effects of the mutant factors themselves. This effect appears 

 to be cumulative, for the presence of several mutant factors often greatly 

 accentuates it. The difficulty has often proven a very great obstacle in 

 carrying out some Drosophila experiments, but it serves to demonstrate 

 that sterility may be a consequence of certain combinations of factors. 



Specifically a number of definite cases may be given. Muller at- 

 tempted to unite the factors for yellow body color, white eyes, abnormal 

 abdomen, bifid wings, vermilion eyes, miniature wings, sable body color, 

 rudimentary wings, and forked spines in one strain of flies. Here, of 

 course, the factor for rudimentary wings in itself might be expected to 

 have a profound effect upon the fertility of the strain, but aside from 

 this effect it was found that the strain was so deficient in viability and 

 general vigor that it was necessary to propagate it by specially devised 

 breeding methods in the heterozygous condition. The heterozygous 

 flies showed only an insignificant reduction in viability and fertility, 

 whereas their full brothers and sisters which were homozygous for the 

 recessive factors were so weak as to be of no value in the experiments. 

 The same difficulties were met with -in dealing with combinations of 

 recessive factors belonging to other groups. It is safe to say that almost 

 any combination of several recessive factors in Drosophila results in 

 diminished vigor and consequent decrease in fertility. The effect is, 

 however, specific, for the degree of diminution depends not only upon 

 the number of recessive factors which are combined, but also upon the 

 specific effects of the factors themselves. The specific residual effects 



