334 J, A DETLEFSEN AND E. ROBERTS 
dropped much earlier, but the effects of selection were conspicu- 
ous and prompted us to carry the experiments through to their 
logical end. 
"WTienever one crosses an individual with the linked factors, 
AB, to the double recessive, ab, then the heterozygote, ABab, 
will form four sorts of gametes: AB + ab, the parental types, 
and aB + Ab, the recombinations or crossovers. The relative 
frequencies of these gametes mil depend upon the distance 
between the loci for A and B, at least according to the commonly 
accepted hypothesis. If a distance on the chromosome, which 
gives 1 per cent of crossovers is adopted as an arbitrary unit, 
then the distance between genes on a chromosome may be deter- 
mined in terms of this arbitrary unit, and the map of a chromo- 
some may be plotted, as has been done by investigators working 
with Drosophila. Repeated trials using large numbers, with 
comparable stocks and controlled envnonmental conditions, 
have shown that the ratio of crossovers to total gametes is uni- 
form enough to suggest that the distance between two genes is 
fairly constant. However, the phenomenon of crossing over is 
not as simple as was first supposed, for a number of genetic and 
environmental influences have been found to affect crossing over 
markedly, at least in Drosophila melanogaster. Bridges ('15) 
stated that crossing over varied with age, for second broods 
showed a rather consistent decrease. Plough ('17) found that 
low and high temperatures (below 17.5°C., and above 28°C.) 
increase the amount of crossing over. Sturtevant ('19) 
found in the second chromosome of Drosophila one gene to the 
left of purple and one to the right, both of which lower the per- 
centage of crossing over in that portion of the chromosome in 
which they He. He also found a similar factor in the third chro- 
mosome. Furthermore, an incompletely investigated case dis- 
closed a dominant third chromosome gene which increased the 
amount of crossing over between purple and curved in the second 
chromosome. Gowen ('19) measured the amount of variability 
shown in a population of 240 Drosophila females with respect to 
crossing over between fixed points in the third chromosomes 
and found a very high degree of variability. His data show that 
