THE MANNER OF OCCURRENCE OF MUTATION 449 
Bridges’ unpublished data show that the locus of scute is 0.0, 
and that of echinus 4.0. The above data therefore give the 
locus of ivory as probably a little less than 1.8, which is a sufh- 
ciently close agreement with the value 1.1 determined for the 
locus of white (Morgan and Bridges, ’16). 
D. Time at which the mutation occurred 
The mutation that gave rise to ivory must have occurred in 
the mother fly of culture 4711. That the ivory males in that 
culture were not due to contamination is shown by the fact that 
they carried the expected second chromosome genes and by the 
fact that ivory is a gene not known to have existed before, so 
that there is no known source from which such contamination 
could have come. That the original ivory males were not due 
to mutation in a preceding generation is shown by their small 
number—9 in 141 males. Clearly the mother was not heter- 
ozygous for ivory in all her oogonia, unless the gene was linked 
to a lethal that killed about 130 ivory males. But the lethal view 
is negatived by the fact that the sex-ratio was 1399: 141 7— 
clearly no sex-linked lethal was present. The mutation must, then, 
have occurred somewhere in the germ tract of the female of 4711. 
We may further conclude that the mutation must have been 
soon after the separation of her germ cells from her somatic 
cells, as it probably affected 13 per cent of the oogonia. This 
percentage is deduced as follows: From any heterozygous unre- 
duced egg the chances are even that the ivory-bearing X will 
pass into a polar body at reduction or will remain in the egg. 
Since it remained in the egg in nine cases where a Y-bearing 
sperm fertilized the egg, the chances are that it passed into a 
polar body in nine more eggs that gave rise to males. That is, 
18 
141 
sperm probably carried an ivory gene before reduction. This per- 
= 13 per cent of the eggs that were fertilized by Y-bearing 
centage, corresponding nearly to the fraction * suggests that the 
mutation occurred in one of the first eight germ cells. Perhaps 
