120 



THE AMERICAN NATURALIST 



[Vol. XLIX 



PP. This must be due to a deficiency in the homozygous 

 RR flies. In other words, the results taken all together 

 (Figs. 1 and 2) show that the disturbance is brought 

 about by factors (in the third chromosome) which in the 

 homozygous state act as lethal s or perhaps as semi- 

 lethals. Eandom introduction of one or two or no lethals 

 may be assumed, as follows : 



(A) If the lethal is introduced by the " pink-bearing" 

 cbromosome, the homozygous pink will be depressed in 

 the F 2 . 



(B) If introduced by the "red-bearing" chromosome, 

 the homozygous red will be depressed in the F 2 . 



(C) If two lethals, both of which are identical, are in- 

 troduced at the same time, one by the red and one by the 

 pink, all classes will be equally depressed, 9 and the re- 

 sults as far as concerns the F 2 ratio will be the same as if 

 there were no lethals present, i. e., the 3 : 1 ratio will be 

 realized. 



(D) If two lethals that are different are introduced 

 at the same time, one by the red and one by the pink, both 

 the homozygous classes (RR and PP) will be depressed, 

 but not the RP. There would be somewhat fewer pinks 

 than expected in the F 2 . 



I wish to acknowledge my indebtedness to Professor 

 T. H. Morgan, whose kind attention and suggestions both 

 throughout the foregoing experiments and in the prep- 

 aration of the present report, were invaluable. I also 

 wish to express my appreciation to Mr. H. J. Muller to 

 whom I owe some suggestions concerning the interpreta- 

 tion of the results. 



Morgan, T. H. An Attempt to Analyze the Constitution of the Chromo- 

 somes on the Basis of Sex-Limited Inheritance in Drosophila. Jour. 

 Exp. Zool., Vol. 11, No. 4. 

 Two Sex-Linked Lethal Factors in Drosophila and their Influence on the 

 Sex-Ratio. Jour. Exp. Zool, Vol. 17, No. 1, July, 1914. 



•LITERATURE CITED 



Hyde, R. R. Fertility and Sterility in Drc 

 Exp. Zool, Vol 37, No. 2, August, 1914. 



in Drosophila ampelophila. Jour. 



