318 
GENETICS: C. B. BRIDGES 
It was found that the chromosome in which the third independent mutation 
to maroon had occurred was carrying two other mutant genes : One of these, 
'dwarf/ is a recessive that reduces the size of flies to only about half that of 
normal sibs; it also sterilizes females homozygous for it. The locus of dwarf 
is within a unit to the right of that of maroon. The other gene causes no 
observable somatic difference, but profoundly affects the process of crossing 
over in third chromosome in definite regions and by definite amounts. A dis- 
tinct new system of cross-over values corresponds to the heterozygous and still 
another to the homozygous condition for the gene. A comparison of the be- 
havior and effects of this gene with that of the already known genetic varition 
called 'Cm' showed that they were probably identical mutations. There was 
found to be 5.9% of crossing over between dichaete and maroon in femal s 
heterozygous for Cm, and 6.6% when females were homozygous for Cm. In 
the region near dichaete there is, as these values show, no great difference be- 
tween the homozygous and heterozygous Cm conditions. 
1 A full account of maroon with the detailed data on which the various statements of 
this paper are based will appear as a section of a publicatian by the Carnegie Institution 
of Washington. 
2 Sturtevant, Science, New York, N. S., 36, 1913, (990-992). 
^Morgan, Ibid., 36, 1912, (718-720). 
