) any Z 
IY 
\ v 
\ ¥ 
v 
554 W? 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. Thisis 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 
Digitized by Microsoft® 
