30 



AN ANALYSIS OF THE EFFECT OF SELECTION. 



constitution, but there are also the crossover eggs, D' l m and -f. 

 If we let the non-crossovers be to the crossovers as x : y, the result of 

 the mating will be: 



D'lm 



- 



= dies. 



dies. 



hn 



The result then is: 

 2 



Per cent crossovers: 



D' i/ = not-D' 



100y^200 (not-DQ 

 : z+2/~D'+not-D' 



In the present case this formula gives the crossover percentage 

 as 0.29. Lethal III is, then, located 0.29 to the right of Dichset. 



Another lethal of the same sort as the one just described appeared 

 in culture 1546. This culture belonged to the sixth generation of the 

 same line in which the first lethal was found, and was descended from 

 a sister pair (1213) to 1264, the first culture in which that lethal ap- 

 peared. Since the two lethals are certainly distinct, as will appear 

 below, this relationship is to be regarded only as a coincidence. Three 

 cultures of this strain were made 1546 and two daughter pairs. 

 The result was 154 Dichsets and 1 not-Dichset. The 1 not-Dichset 

 was from culture 1681. The Dichsets from this culture show both 

 parents to have had the constitution 



iy 



s e l m s t e a r 



The not-Dichset individual was spineless, sooty, rough. This indi- 

 cates that the lethal was to the left of Dichset; otherwise the egg in 

 question must have resulted from a sepia Dichaet spineless triple cross- 

 over, which is a very rare occurrence. By the method outlined above 

 it may be calculated that the lethal gives 1.29 per cent of crossovers 

 with Dichset. 



That these two lethals are distinct is indicated by the following 

 culture, 1915. The female of this mating came from culture 1791, 

 which gave 31 Dichsets and no not-Dichsets. 1791 was an F 2 from a 

 cross involving 1419 of the 1264 line, and thus its lethal must be sup- 

 posed to be that of 1264. The male of the test bottle 1915 was from 

 1681 of the second lethal strain. Therefore, if the two lethals are the 

 same, 1915 should have given few or no not-Dichsets; if they are dif- 

 ferent it should have given 2 Dichsets to 1 not-Dichset. The actual 



