304 



THE AMERICAN NATURALIST 



[Vol. L 



which appear streaked but otherwise normal, must have 

 the composition: ^ These are the "F/' 



females whose gametic output is to be tested. Accord- 

 ingly, they are crossed to normal males. All the offspring 

 ("F 2 ") appear normal (except for the dominant, streak), 

 but the factors they received from their mother may be 

 determined by mating them, individually, to % males, 

 for the latter contain (heterozygous) all recessive charac- 

 ters possible in the former. 



It was at first thought that labor might be saved, and 

 certain points in addition determined, by conducting the 

 linkage determinations on flies heterozygous for the fac- 

 tors used in both chromosomes I and II at the same time, 

 instead of making determinations of the linkage in the 

 two chromosomes in separate experiments. The multiple 

 stocks of the two chromosomes were, therefore, crossed 

 together, and females were finally obtained that had the 

 composition : 



ywAbvmsrf Str b, p, v g a r s P 

 ec, B r oUj Cv ba 



These females, heterozygous for 22 mutant factors, were 

 then crossed to normal males, and the composition of their 

 female offspring was tested by mating these in separate 

 bottles to % males. The maintenance of the double- 

 multiple stocks proved to be extremely difficult, however, 

 and so, after obtaining determinations for 166 offspring 

 from such females, the two groups of mutant factors were 

 again separated. The data obtained in this part of the 

 experiment show that there is no linkage of any of the 

 twelve factors studied in group I with any of the ten 

 studied in group II; this is of course in marked contrast 

 to the relations shown between factors in the same group. 

 The conclusions of previous workers that no factor in one 

 group was linked with any factor in another group were 

 based on results obtained with comparatively few com- 

 binations of factors, which were chosen as samples, so 

 to speak. It will be seen that in the present work these 

 conclusions have been confirmed by a study of 132 differ- 



