128 
GENETICS: C. B. BRIDGES 
Proc. N. A. S. 
be considered. For each chromosome and for each region within a chromo- 
some the amount of this multiple crossing over is characteristic, and may 
not be the same in amount for different sections of equal map-distance. 
Because of this variation, the accurate expression of the relation between 
map-distance and exchange-values for the more distant loci, requires a 
table of conversion corrections for each pair of loci considered. Such 
tables will be published with the more detailed maps of the chromosomes. 
In general, the correction is relatively slight with distances that are under 
20-40, but for longer intervals the correction increases at an accelerated 
rate. We have not met with percentages of exchange that exceed 50.0, 
though two of the maps are about a hundred units long. 
The map of the third chromosome is the most accurate, since the cal- 
culation of the distances between the principal loci is made on the basis 
of all data up to 1920, and an improved method of weighting and inter- 
relating the data has been followed. Relatively little change in these 
primary distances — the "triangulation" of the map — is expected with the 
further accumulation of data. There is still some uncertainty with re- 
gard to the region to the left of spineless, for the different sets of data 
upon that region may not be comparable because of the possible presence 
of crossover variations. The data used in the calculations for the primary 
distances in the first and second chromosomes are homogeneous, and al- 
though not including the last four years' work, are still fairly ample in 
amount. Changes are to be expected when these two maps are recalcu- 
lated with the improved method and complete data. 
The most useful mutants are those that are separable from the wild- 
type with completeness and ease, that are not inferior to the wild-type 
in viability and productivity, and that do not interfere with the use in 
the same experiment of any large class of the other mutant characters. 
Dominants are more valuable than recessives. Mutants accurately 
located in the chromosome are more valuable than those whose positions 
are less well established, though if the other desirable features mentioned 
above are present in a given new mutant the position will be found rather 
quickly. A very large factor in the value of the mutant is its position 
in the chromosome. The end positions are most valuable. Throughout 
the remainder of the chromosome the most favorable positions are those 
evenly spaced and just close enough together so that no double crossing 
over occurs between them. If the interval is too small there is trouble 
in getting double recessives, and the crossover classes are so small that 
large totals are required to make differences significant. 
The mutants that fulfill all the above requirements most nearly are 
followed by an asterisk (*) in the maps. Their loci are the primary bases 
with relation to which the other mutants are located. There are several 
cases in which two or more excellent mutants affecting quite different 
