Vol. 6, 1920 
GENETICS: J. A. DETLEFSEN 
669 
parent; while the highest 12 have values above 21% and cover about the 
same range as the high parent. The values for both low and high F2 
variates are based upon totals which are just as satisfactory as any in the 
population, where the average number of progeny per female was 260. 
The total Fi included 61,000 offspring and the total F2 distribution is 
based upon 38,500 offspring. 
TABLE 4 
The High and Low Variates of the Total Fi Distribution in Table 2 
LOW VARIATES HIGH VARIATES 
RECORD NUMBER 
CROSS- . 
OVERS 
TOTAL = 
CROSSOVER 
VALUE 
RECORD NUMBER 
CROSS- . 
OVERS 
TOTAL = 
CROSSOVER 
VALUE 
2- 2 
1 
35 
— 2 
.86 
2-32 
66 
287 
23.00 
2- 6 
18 
216 
- 8 
33 
5-21 
52 
154 
33.77 
2-19 
14 
223 
- 6 
28 
5-22 
45 
178 
25.28 
2-22 
0 
19 
= 0 
00 
5-24 
69 
325 
21 .23 
2-25 
8 
168 
= 4 
76 
5-28 
112 
466 
24.03 
2-28 
14 
174 
= 8 
05 
6-10 
107 
416 
25.72 
2-33 
22 
329 
- 6 
69 
6-21 
56 
241 
23.24 
2-34 
25 
280 
= 8 
93 
6-23 
71 
287 
24.74 
6- 8 
11 
281 
= 3 
91 
6-24 
61 
251 
24.30 
6- la 
32 
394 
= 8 
12 
6-1 la 
33 
145 
22.76 
6- 2a 
23 
275 
- 8 
36 
6-13a 
26 
89 
29.21 
6-35a 
20 
272 
35 
6-16a 
62 
295 
21 .02 
Total 
188 
2666 
05 
Total 
760 
3134 
24.25 
In obtaining a crossover value for any two genes like white and miniature 
we find much variability among the females which serve to make up the 
general population from which our map value is derived. This variability 
is due to numerous modifying factors. Selection has evidently sifted out 
certain relatively pure combinations of these modifiers, hence the low 
variability of our low crossover stock. The hybridization experiments 
indicate that the amount of crossing over is at least markedly influenced 
if indeed it is not actually determined by multiple factors. There are 
several ways in which multiple factors might possibly change the crossover 
value which two genes show. In modifying the crossover value of white 
and miniature from 33% to 6% or to 0% we might suppose that we had 
either moved the locus of genes or that we had eliminated the usual single 
chromosomal twist between these two genes. Since the allelomorphic 
relationships between red and white and between long and miniature 
have not been disturbed when we mate low crossover stock to the original 
population, the latter alternative explanation seems preferable. We 
can evidently change by selection the amount of twisting which members 
of an homologous pair of chromosomes show. Now, if the difference 
between practically no crossing over (Series A and A') or between 6% 
crossing over (Series B) and normal crossing over (33%) is due to multiple 
