664 
GENETICS: J. A. DETLEFSEN 
Proc. N. a. S. 
We do not know that the distance which gives 1% (or n%) of crossovers 
is a fixed unit. Stated differently, we do not know how constant the 
percentage of crossing over may be between two genes to which we give 
a fixed distance, i. e., our arbitrary unit of measurement may itself prove 
to be a variable. It may be possible for the distance which gives 1% of 
crossing over to differ in different females of the same population, or 
differ between stocks. In order to throw some light on these questions 
I began a set of experiemnts in 1916 in collaboration with my colleague, 
Dr. B. Roberts, and several students. Although a number of these experi- 
ments are still in progress, data involving the classification of over 400,000 
individuals have been accumulated and some conclusions seem warranted. 
A more detailed account of these experiments will appear in the current 
.numbers of the Journal of Experimental Zoology, 
In observing a large number of females (Drosophila melanogaster) of 
the generalized zygotic formula ^ ^ , it is common to find great differ- 
ences between individual females with respect to the amount of crossing 
over. Some of the variability may be due to sheer fluctuations of sampling, 
to age, and to environmental conditions, but sometimes the deviations are 
so wide as to arouse a suspicion that hitherto unknown causes may be 
effective. If this variability is due at least in part to genetic modifiers 
then selection should be effective, particularly if environmental fluctua- 
tions do not mask or obliterate the effects of genetic modifiers. It was 
with this thought in mind that I began to select for high and low cross- 
over value. Four selection experiments were undertaken. Series A, 
A ' and B were low selection experiments and Series C was a high selection 
experiment. Each series began with a single white-eyed, miniature- 
winged female mated to a wild, red, long male. The Fi females were red, 
long double heterogzyotes ^ ^ and the F\ males were white, miniature 
double recessives These were mated in pairs, giving the parental 
classes (red long and white miniature) and the crossovers (red miniature 
and white long) with the usual ratio of approximately 33% crossovers — 
the same value used in plotting chromosome maps. The same mating 
was made in successive generations, always selecting as far as was possible 
the widest deviates to perpetuate the line of selection. At the same time, 
the closest possible inbreeding was maintained, the details of which are 
given in our longer papers. The results indicate that selection was 
effective in all series. 
Series A was reduced to 0% in Fio and remained at about 0% for two 
more generations. Series A', derived from Series A as a side line of selec- 
tion in the Fy, began with a female showing 1 : 91 = 1.10%.^ This 
line was carried for 9 generations (F7-F15) and also bred true to about 0%. 
The grand total for this entire series gave 33 : 5156 = 0.64% — actually 
