GENETICS: H. H. PLOUGH 
167 
LINEAR ARRANGEMENT OF GENES AND DOUBLE CROSSING OVER 
By Harold H. Plough 
Department of Biology, Amherst College 
Communicated by T. H. Morgan, April 11, 1919 
In his recent criticism of the theory of hnear arrangement of the genes in 
the chromosome, Prof. W. E. Castle states that that theory calls for a num- 
ber of unproved secondary hypotheses, among which is the assumption of 
TABLE 1 
NUMBER 
TEMPERATURE 
TOTAL 
NUMBER 
PER CENT 
CROSSOVERS 
PER CENT 
INCREASE OVER 
CONTROL 
PER CENT 
CROSSOVERS 
PER CENT 
INCREASE OVER 
CONTROL 
b pr c 
b-pr 
pr-c 
1 
2 
3 
4 (Control) 
5 
6 
7 
deg. C. 
9.0 
13.0 
17.5 
22.0 
29.0 
31.0 
32.0 
995 
2,972 
2,870 
15,000 
4,269 
3,547 
4,376 
13.5 
13.5 
8.3 
6.0 
8.8 
14.0 
15.7 
125 
125 
38 
47 
133 
162 
25.8 
27.2 
23.0 
19.6 
22.5 
26.7 
26.5 
32 
39 
17 
14 
36 
35 
SI 
b c 
S^-b 
b-c 
8 (Control 1 
9 
22.0 
31.5 
6,009 
3,769 
38.4 
37.8 
Decrease 
21.1 
33.2 
58 
b pr vg 
b-pr 
pr-vg 
10 (Control) 
11 
22.0 
31.5 
2,139 
1,099 
10.0 
15.1 
50 
15.1 
20.2 
34 
Data from H. H. Plough, /. Exp. ZooL, 24, No. 2. 
double crossing over. There are, however, a number of facts, each inde- 
pendent of the original hypothesis, that require double crossing over in their 
explanation. Some time ago^ I demonstrated that treatment of female flies 
with temperatures both above and below the 'normal' (20°-27°C.) resulted 
in an increase in the percentage of crossing over in the second chromosome. 
This increase for a short chromosome 'distance' was roughly proportional to 
the number of degrees above or below the 'normal.' Bridges^ had already 
shown that age increased crossing over and more recently it has appeared 
probable that other environmental agents may cause a similar increase. In 
all cases, as pointed out in the original paper, the increase is much greater for 
short regions than for longer ones, and finally, for genes which normally 
