26 AN ANALYSIS OF THE EFFECT OF SELECTION. 
bristle number of the two classes of Dichzt offspring and the differ- 
ence between these two means. ‘The sign of the difference is given as 
positive when the not-specks had a higher mean than the specks (as 
in all these cases); negative, as in other results (see below), when 
the specks were higher. In the col- 
umn headed oe. is given the quotient 
of the difference between the two means 
divided by its own probable error, a 
measure of the probable significance of 
that difference. The last two columns 
give the x? and P values for the two 
(speck and not speck) distributions, 
considered as wholes. 
These data make it certain that the 
864 plus line and crossbred plus line 
both contained second chromosomes 
with one or more plus modifiers domi- 
nant to minus modifiers in the second 
chromosome of the speck stock. The 
1002 plus line had similar modifiers, 
and also had the same relation to the 
1331 minus line. It is probable, from 
the results obtained with the 1002 line, 
that the speck stock and the 1331 line 
had some minus modifiers in common, Ay oR 
but that the second chromosome of the ee Fee vio) aoe ee 
1331 “line -was-more~ strongly-minus. —... "ale bere ee one 
Both these latter results would have based on 153 not speck and 106 speck 
been expected, since the second chro- _ Mies, are both reduced to the percent- 
® 3 age basis. See table 19 for statistical 
mosome of the 1331 line came, in part treatment. 
at least, from that of the speck stock, 
but has been minus selected, while the speck stock has not been 
selected at all for bristle number. 
The experiments just discussed show that the second chromosome 
contains one or more modifiers, but give us no information regarding 
the loci of such modifiers. It is possible, on the basis of this data 
alone, that speck itself is the minus modifier. If, however, a heterozy- 
gous female is tested by mating to a speck not-Dichxt male, there 
will be a possibility of crossing over between speck and any modifiers 
in the second chromosome. The result would be that the speck and 
not-speck offspring differ less than when an F, male is tested. There 
is, of course, also an opportunity for crossing over in the third chromo- 
somes of such females, so that the Dichet offspring will not be all 
alike with respect to their third chromosomes, as they were when the 
male was tested; but the same crossover classes should occur among 
