AN ANALYSIS OF THE EFFECT OF SELECTION. 31 
result was 51 Dichets to 30 not-Dichets. Evidently, then, the two 
lethals are distinct, as was previously indicated by the fact that they 
are probably on different sides of Dichet. 
It seemed possible at first that one or both of these lethals might 
be due to a breaking up of the Dichset factor, whereby its lethal effect 
had been separated from the effect it produces on the soma of a het- 
erozygous fly. This hypothesis is negatived by two considerations: 
(1) both lethals have been shown to occupy loci different from that 
for Dichxt; (2) the lethal effect of Dichet is not allelomorphic to that 
of these factors, since a fly with Dichet in one chromosome and either 
of the lethals in its mate does not die. 
EXTENDED. 
In culture 1379, of the crossbred plus series, there appeared several 
flies intermediate in appearance between Dichet and the normal. 
These flies had the bristles of the normal flies (including the anterior 
post-alars, always reduced or absent in Dichets), but had their 
wings spread out to a greater or less extent. These individuals were 
tested, and were found to have a dominant factor, responsible for the 
extended wing character. The character has been called “Extended” 
(see plate 1, fig. 1). It occasionally overlaps the normal, and is there- 
fore not favorable for linkage experiments. It is, however, sufficiently 
uniform in appearance to make it possible to work out its inheritance 
with certainty. The gene is found to be an allelomorph of Dichet, 
and is designated D®. Like Dichet, it is lethal when homozygous; 
and the flies with Dichet in one chromosome and Extended in the 
other also die. These conclusions are based on the following results: 
Preliminary experiments involving speck (chromosome IT) and 
various characters in chromosome III showed that Extended crosses 
over freely from speck in the male, but gives apparently no crossing 
over in the male with sepia, spineless, or rough. These data are not 
very satisfactory, owing to the fact that some of the Extended flies 
are very similar in appearance to the not-Extended, and there is too 
great an opportunity for being influenced by the other characters of 
the flies when making the separation. However, no crossovers were 
discovered among 308 flies. 
When tests were made of heterozygous females, there was found 
to be a slight excess of not-Extended offspring, presumably due to 
incorrect classification. The proportion of crossovers, based on Ex- 
‘ 13 ; 
tended offspring only, was i057 12.4 per cent for sepia Extended 
Val 
and jaa "8 per cent for Extended spineless. In one experiment in 
which all three of these factors were observed at once, the result shown 
in table 23 was obtained. 
