GENETICS: METZ AND BRIDGES 
675 
cific for the two mutants in question, and is not an ordinary case of 
sterility, is shown by the fact that both mutants are fertile and give 
fertile hybrids with other mutants and with the normal. Furthermore, 
the sterility of the hybrids in question is in no way dependent upon the 
males used, at least so far as normal, rugose and glazed are concerned, for 
all three kinds have been tried. 
2. In Drosophila Melanogaster (Ampelophila) . — The case found in 
D. melanogaster may be, summarized as follows: A mutant form, 'notch' 
wing, crossed to the fully fertile mutant 'facet' eye, gave Fi hybrids 
which were completely sterile with the common parent type and with 
one of the mutant forms, facet. 
The sex-linked mutant, notch, is principally characterized by a notch 
of definite type but variable extent in the tip of the wing. Notch is 
dominant in the female, and in the male acts as a lethal. The locus of 
notch was found to be at approximately 2.6.^ In an attempt to locate 
the gene for notch more accurately, it was decided to use in a linkage ex- 
periment the recessive mutation facet^ whose locus had been found to be 
at approximately 2.2, very close to the supposed position of notch. Ac- 
cordingly, when notch reappeared (December, 1915), the single notch 
female, which appeared as a mutation in a pedigreed culture, was mated 
to yellow facet males (yellow is a recessive body color whose locus is at 
0.0). 
The Fi generation furnished a surprise, for every one of the daughters 
that was notch was also facet! Two explanations of this peculiar 'com- 
pounding' of notch and facet are open to us: either the notch mutation 
is a deficiency* for facet, or the two mutant forms are allelomorphic. 
It had been the intention to backcross a number of the expected Fi 
notch females to yellow facet males in order to secure sufficient linkage 
data upon the relative positions of these factors. When it was found 
that facet showed an apparent dominance with notch this plan pre- 
sented difficulties. Nevertheless, nine of the notch-facet daughters 
were mated to yellow facet males as originally planned, while six more 
were mated to yellow notch-facet males. Not one of these fifteen females 
produced offspring. It was then realized that this relation was unusual, 
and a second effort was made to secure offspring. Fresh males were 
given to the remaining females (three having died) and they were trans- 
ferred to fresh culture bottles. The discarded parent bottle was also 
rescued and three additional notch-facet females — -the last to hatch — 
were secured and likewise mated to yellow facet males. No offspring 
were produced. Since the death of these females 'without issue' would 
mean the loss of the mutation and would shut the door on the solution 
