32 



AN ANALYSIS OF THE EFFECT OF SELECTION. 



It is evident from these data that Extended is between sepia and 

 spineless, some distance from either. It is, then, in the same general 

 region as Dichaet. 



The lethal effect of Extended has been tested in two ways. Mat- 

 ings of Extended by Extended gave 116 Extended to 94 normals. 

 If homozygous Extended is viable the result should be 3 : 1 ; if it 

 dies the result should be 2 : 1. In fact, it was nearer 1:1. This 

 result is probably due to the overlapping phenomenon, resulting in the 

 classification of some Extended flies as normal. It is suggestive of a 

 2 : 1 ratio, however. More conclusive data was obtained by mating 

 heterozygous Extended to Dichaet flies heterozygous for lethal III 

 (see above), and inbreeding the Extended offspring. If Extended is 

 lethal when homozygous, these flies should produce only Extended 

 offspring, but these should all be heterozygous. They should, in fact, 

 breed exactly like the true-breeding race of Dichaets described above. 

 This is actually the case. Such a stock has now been kept for four 

 months, and is still made up almost entirely of evidently Extended flies; 

 but tests show them to be only heterozygous for the character. 



TABLE 23. 



The mating of DichaetX Extended (or vice versa) gave the following 

 result: Dichset, 99; Extended, 69; normal, 102; total, 270. If we 

 suppose some of the flies classified as "normal" to be in reality Ex- 

 tended, this result approximates to the 1:1:1 expected if Dichaet- 

 Extended flies die. . The fact that the Dichaets are only about a third 

 of the total shows that half the Dichaet gametes have been eliminated 

 somehow. One of the Dichaets and a number (4 individual matings 

 and 2 mass cultures) of the Extendeds have been tested, and neither 

 sort has produced the other. It is, then, safe to conclude that Dichaet- 

 Extended flies die. 



Culture 1379, in which Extended first appeared, was made up by 

 mating together two 8-bristled flies, the male from 1145, the female 

 from 1253. The latter culture gave among other offspring 5 sevens 

 and 2 eights. The other eight, in 1356, behaved normally, as did 

 one of the sevens (in 1357). Culture 1145, however, gave no seven 

 and only the single eight. Since 1379 gave a result indicating that one 

 parent was Extended instead of 8-bristled Dichaet, it seems probable 

 that the male parent, from 1145, was the mutant. In either case, 

 the Extended parent was produced by mating a 7-bristled Dichset 



