GENETICS: C. B. BRIDGES 
317 
same property of specific recurrent mutation has since been found to be char- 
acteristic of several other loci, of which notch, vermilion, rudimentary, and cut 
are the most striking examples. 
At this time (April, 1912) pink was the only mutation known to be in the 
third chromosome (by definition) so that the F 2 results from the cross of ma- 
roon to pink constituted the only test as to whether maroon also is in the third 
chromosome. Maroon differs enough from pink in its characteristics so that 
the separation of the two forms could be accurate for about 85% of the in- 
dividuals. The Fi flies from the cross were wild- type (two non-allelomorphic 
recessives), and the F 2 flies were 485 wild- type: 215 maroon-like: 183 pink- 
like : 0 maroon pink. 
It was expected that the flies which were homozygous for both maroon and 
pink would be as much lighter than pink as maroon is lighter than the wild- 
type, since this was the usual type of relationship. No flies lighter than pink 
were found, but it was uncertain whether this meant that the double recessive 
was actually absent because of strong linkage (no crossing over), or that the 
double form was present but indistinguishable from pink or from maroon. 
That the double form should be present, but not lighter than pink, would mean 
that maroon and pink are 'non-modifiers' of each other. Subsequent work has 
shown that' 'non-modification' is the usual relation between the different pinks 
that have arisen. 
That the apparent absence of the maroon pink class was a real absence due 
to linkage was soon proved: Tests, by means of F 2 and back cross results, of 
the linkage relations between maroon and the second chromosome mutant 
"arc" showed that these two genes assort with complete independence. The 
above tests showed that the locus of maroon was probably not in the second 
chromosome, and was therefore probably in the third chromosome. Mean- 
while the body color ebony had arisen and had been shown to be linked to 
pink (Sturtevant, '13). 2 Since there could be no question of failure of identi- 
fication of the double recessive maroon ebony the next step was to show that 
maroon is linked to ebony. The F 2 of the cross of maroon by ebony gave a 
2:1 : 1 : 0 linkage ratio, which we had just learned to explain as the result of 
'no crossing over in the male' (Morgan, '12). 3 The F 2 from the maroon by 
pink cross was thus proved to have been a 2 : 1 : 1 : 0 ratio, and was in fact 
the first such linkage result obtained for the third chromosome. 
The location within the third chromosome of the gene for maroon was made 
easy and direct by the discovery and location of two excellent III chromosome 
dominants, 'dichaete' and 'hairless.' The locus of dichaete is quite near the 
left end of the third chromosome (at about 11.0) while that of hairless is near 
the middle (at about 32.0). The back cross tests showed that the locus of 
maroon is to the right of dichaete by about 4.2 units, or is at a position of 15.2 
(11.0 + 4.2) when referred to the locus of sepia as the zero point. With hair- 
less, maroon gave 21.2% of crossing over, which corresponds to the location 
of maroon between dichaete and hairless. 
