666 
GENETICS: J. A. DETLEFSEN 
Proc. N. a. S. 
repeat the experiment and test out the region between white and miniature 
in such females. 
The effects of selection on crossover values may be due to one or a num- 
ber of causes, some of which suggest themselves almost immediately. 
The most promising and probable explanation seemed to me to be that 
crossing over is either due to or markedly modified by multiple factors. 
In order to test out this view, I crossed the low crossover stock of Series 
B, which shows 5-6% crossing over, to ordinary stock which shows 33% 
crossing over. Table 2 gives the results of this experiment. The first 
line of the table gives for comparison the frequency distribution of cross- 
over values in an ordinary population. There were 90 females in this 
sample but I have eliminated two very wide deviates from the distribution, 
because the number of offspring on which their crossover values were 
based was extremely small. One of them showed 1:8= 12.5%; and the 
other showed 6:8 = 75%. The population as a whole showed 30.68% 
crossing over and the mean female had a value of 30.55%. The average 
number of offspring per female and the totals show that the values for the 
females both individually and collectively are as reliable as can be reason- 
ably expected. The second line of the table shows the first generation 
in Series B, which resembles closely the sample just described. After 28 
generations, selection was discontinued. Thereafter the generations were 
perpetuated by en masse matings. In the F42, I mated 50 red, long females 
heterozygous in white miniature ^ ^ to stock white miniature males 
w_ni. All except one were fertile and the distribution of their crossover 
values is given as the low Pi parent in table 2. Mating them to ordinary 
stock males would not change the crossover values which such low females 
show. We cannot know positively what the crossover value of each white 
miniature male parent was, but we have no Reason to suppose it differs 
greatly from the values given for the first generation of Series B or the 
ordinary stock, both shown in table 2. We have used this same white 
miniature stock in class work and have always found it to give the regular 
"map value" of about 33%. 
It was virtually impossible to breed all of the Fi hybrid females from 
each pair separately. I decided to breed exhaustively 50 red long Fi 
vu fyi 
females ^ ^ to their Fi white miniature brothers w m to obtain a 
sample frequency distribution of Fi females coming from a single Pi pair 
(pair No. 18). Forty-seven of these 50 females were fertile and gave an 
average of 465 offspring per female.^ The range of Fi crossover values 
shows quite clearly that they lie between the low and the high parents. 
The value of the mean female and the crossover value of this total Fi 
population show the same thing. One Fi female showed a ratio of 1 : 36 = 
2.77%, but since the ratio is based upon such a small total we need not 
lay much stress on this wide variate. 
