130 
GENETICS: C. B. BRIDGES 
Proc. N. A. S. 
regions of the body are located very close to one another (e. g., yellow and 
scute, sable and garnet, forked and bar, sepia and hairy, hairless and 
ebony, claret and minute). Of these neighboring mutants that one is 
used which interferes least with the other characters in the particular 
experiment, and the results of all such experiments are correlated by a 
simple correction made possible by knowledge of the distance between the 
loci. Such groups of loci, the members of which can be used as alternates, 
are treated as one base in the construction of maps and in linkage work. 
The mutants followed by a plus sign (+) are somewhat less valuable 
in one or another respect, perhaps only in that of position. 
Of the remaining mutants, not marked, some are exceedingly valuable 
in special work. As an example of these may be mentioned CIII, which 
prevents practically all crossing over within the right half of the third 
chromosome and thus is much used in holding other mutants in their 
proper relations in balanced stocks and while making up multiple stocks. 
But most of the unmarked mutants are those more difficult to separate, 
of poor viability, or less accurately located. Thus the mutant bithorax, 
which is located very close to spineless, has second rank and would be 
used as an alternate to spineless if the relation between the loci were 
accurately known. But the loci of these two mutants are so close to- 
gether that as yet no double recessive has been obtained, and hence no 
back-cross test of the order and distance between these two loci has been 
made. Dominants, and recessives close to dominants, are easily and 
accurately placed, but there may be an error of five or even more units 
in the indicated positions of certain (unmarked or omitted) second- 
chromosome recessives whose positions are as yet known Only through 
reference to a distant base. These are relatively unimportant mutants, 
and since they are not very useful, there is little incentive to determine 
their locations more accurately. The serial order of the loci marked 
with an asterisk is certain, and in only a few cases are the positions of the 
second-rank mutants likely to be shifted with the accumulation of data. 
Most of the unmarked mutants have their proper position with respect 
to the first and even second rank mutants, but their order with respect to 
each other is often uncertain in the absence of direct tests. 
About 25 of the mapped loci represent more than one mutant allelo- 
morph; thus, there are twelve allelomorphs at the white locus, and three 
to six allelomorphs at each of several other loci. Many of the mutants 
are recurrent; e. g., vermilion, notch, and rudimentary have each arisen 
independently on more than ten occasions. 
It will be noticed that the mutant loci are not distributed at random 
along the chromosome maps, but are more closely spaced at the left end 
of the first, at both ends of the second, and throughout the mid-region 
of the third. This massing may correspond to a like massing of the genetic 
