AN ANALYSIS OF THE EFFECT OF SELECTION. 27 
both the speck and the not-speck offspring, and in identical propor- 
tions. Therefore this factor should not influence the end-result. 
Table 20 shows the results obtained from such experiments. The 
arrangement is the same as in table 19. 
As was expected, the differences here are less than in the correspond- 
ing tests, but are still present and in the same direction when significant. 
This result proves that one or more of the second-chromosome modi- 
fiers cross over from speck in the female. 
TasBLEe 20.—2 Tests, Chromosome IT. 
Source. Means. Distributions. 
Tested |— 
against— Diff. 
Not-sp. sp. Not-sp. sp. Diff. PE. x? Pp 
GS ralcce ster taiats sp. stock 1331 4.971+0.097|4.600+0.082|+0.371+0.127| 2.9 | 4.59 0.21 
SBS a isvciefeccue sexe 1331 sp. stock |4.674+ .059/4.321+ .067/+ .353 .089) 4.0 9.10 .06 
SEF Waveraataialateste 1331 1331 4.448+ .043/4.245+ .044/+ .203+ .062| 3.3 |13.1 .004 
QO 2 Kiaterateheverersts sp. stock | sp. stock |5.543+ .074/5.3934 .104/+ .150+ .128) 1.2 2.78 -43 
ROOD ier rcrtctercreis sp. stock 1331 4.875+ .062/4.787+ .070/+ .088+ .093) 0.9 | 1.39 .92 
TOOZMe ci. :tsy7- ic 1331 1331 4.617+ .046/4.250+ .056/+ .367+ .073) 5.0 |13.8 .03 
LOO He eysitebyaree 1331 sp. stock |5.336+ .027/5.390+ .048]/— .054 .055) 1.0 | 8.93 .26 
Cross-br. minus.| sp. stock 1331 5.259 .097/4.727+ .152/+ .532+ .180} 3.0 | 7.97 .05 
Cross-br. minus.| sp. stock | sp. stock |4.350+ .086|4.609+ .090)— .259+ .125) 2.1 | 2.21 84 
THIRD-CHROMOSOME MODIFIER. 
If we cross two races, one of which is Dichet rough, the other wild- 
type, the F, female will have the constitution D’r,. If such a female 
be mated to a not-Dichet rough male, there will be two types of 
Dichzet offspring—the non-crossovers will be rough, the crossovers 
not rough.! If the two original chromosomes differed in modifying 
factors somewhere near rough, these two types of offspring will differ 
in their bristle number. 
Such tests have been carried out, with the results shown in table 
21.2. In only one case (the third) was a significant difference obtained ; 
but that case proves that there was a dominant plus modifier in the 
1002 line, located somewhere near rough, or a dominant minus modifier 
in the speck stock, but not in the 1331 line and in the same region. 
Since the 1331 line was derived from a cross involving the speck stock, 
and had been minus selected ever since that cross, it is probable that 
dominant minus modifiers present in the speck stock would have been 
preserved in the 1331 line. It is, therefore, almost certain that the 
1 There will be some doubie crossovers, but these will be rare. There will, be course, also be 
two classes of not-Dichets. 
2 In the case recorded in the second row, the Dichet and rough came from different parents, 
so that the non-crossover and crossover classes are reversed. The experiment is the same in 
principle as that outlined above. 
