AN ANALYSIS OF THE EFFECT OF SELECTION. 31 



result was 51 Dichsets to 30 not-Dichsets. Evidently, then, the two 

 lethals are distinct, as was previously indicated by the fact that they 

 are probably on different sides of Dichaet. 



It seemed possible at first that one or both of these lethals might 

 be due to a breaking up of the Dichaet 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 Dichaet; (2) the lethal effect of Dichset is not allelomorphic to that 

 of these factors, since a fly with Dichset 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 Dichset and the normal. 

 These flies had the bristles of the normal flies (including the anterior 

 post-alars, always reduced or absent in Dichsets), 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.^. 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 Dichset, 

 and is designated D^ Like Dichset, it is lethal when homozygous; 

 and the flies with Dichset in one chromosome and Extended in the 

 other also die. These conclusions are based on the following results: 



Preliminary experiments involving speck (chromosome II) 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 777^= 12.4 per cent for sepia Extended 



11 W6 



and 7Ti~'^-^ P®^ ^®^* ^^^ Extended spineless. In one experiment m 



which all three of these factors were observed at once, the result shown 

 in table 23 was obtained. 



