Vol. 6, 1920 
GENETICS: J. A. DETLEFSEN 
665 
less than 1 %. There can be no doubt but that an original crossover 
value of about 33% has been changed by selection, at least a marked 
change has followed selection. 
Series B was entirely independent in its origin from Series A and A^; 
and low selection was also effective, as in the case of the preceding series. 
Curiously enough we have not been able as yet to reduce the crossover 
value to zero, or approximately zero, as in the other series. Since experi- 
ments with this series form the basis of the present paper, table 1 is given 
to show in condensed form the progress during selection. After the F28, 
selection was discontinued, and the stock has bred practically true to 
about 5 or 6% crossing over for 22 generations (i. e., through F50). Table 
1 gives the percentage of crossovers for every fifth generation, and also 
for the total offspring in each block of five successive generations. 
TABLE 1 
Crossover Values for Every Fifth Generation, and for the Totals in each 
Block of Five Successive Generations, in Series B (Low Selection) 
GENERA- 
TIONS 
CROSSOVER 
VALUES 
GENERATIONS 
INCLUDED 
CROSSOVERS 
TOTALS 
CROSSOVER 
VALUES 
1 
28.60 
1- 5 
10517 
40567 
25.93 
5 
24.55 
6-10 
10344 
47295 
21.87 
10 
16.99 
11-15 
3687 
25333 
14.55 
15 
11 .17 
16-20 
4869 
48277 
10.09 
20 
9.81 
21-25 
3386 
36693 
9.23 
25 
7.15 
26-30 
576 
8007 
7.19 
30 
5.62 
31-35 
121 
2089 
6.79 
35 
4.18 
36-40 
267 
4571 
5.84 
40 
6.70 
41-45 
750 
12453 
6.02 
45 
6.51 
46-50 
350 
5203 
6.73 
50 
6.98 
Series C, high selection, was carried for 8 generations, but we were 
unable to make progress in selecting upward. On the contrary, we were 
greatly astonished to find in the F7, 9 out of 72 pairs, which gave almost 
no crossing over at all, although they produced a large number of offspring. 
The remaining 63 pairs gave about the usual crossover value. The totals 
for these 9 paired matings were : 
26 crossovers : 1055 total = 2.46% crossing over. 
These same genes should have given 33% crossing over if they agreed 
with the usual values used in plotting chromosome maps. The natural 
inference is that any attempt to increase crossing over leads to double 
crossing over and thus to very low crossover values (practically zero). 
The explanation implies that these 9 females showed a marked decrease 
in crossover values, despite high selection, because they gave almost 
nothing but double crossovers. Series C was dropped, but we hope to 
