STURTEVANT 



cross-overs really represents distance, 

 AC must be approximateh', cither AB 

 plus BC, or AB minus BC, and not any 

 intermediate value. From purely math- 

 ematical considerations, however, the 

 sum and the difference of the propor- 

 tion of cross-overs between A and B 

 and those between B and C are only 

 Ihnithig values for the proportion of 

 cross-overs between A and C. By using 

 several pairs of factors one should be 

 able to apply this test in several cases. 

 Furthermore, experiments involving 

 three or more sex-linked allelomorphic 

 pairs together should furnish another 

 and perhaps more crucial test of the 

 view. The present paper is a prelimi- 

 nary report of the investigation of 

 these matters. 



I wish to thank Dr. Morgan for his 

 kindness in furnishing me with ma- 

 terial for this investigation, and for his 

 encouragement and the suggestions he 

 has offered during the progress of the 

 work. I have also been greatly helped 

 by numerous discussions of the theo- 

 retical side of the matter with Messrs. 

 H. J. Aluller, E. Altenburg, C. B. 

 Bridges, and others. Mr. Muller's sug- 

 gestions have been especially helpful 

 during the actual preparation of the 

 paper. 



THE SIX FACTORS CONCERNED 



In this paper I shall treat of six sex- 

 linked factors and their inter-relation- 

 ships. These factors I shall discuss in 

 the order in which they seem to be 

 arranged. 



B stands for the black factor. Flies 

 recessive with respect to it (b) have 

 yellow body color. The factor was 

 first described and its inheritance given 

 by Morgan ('11 a). 



C is a factor which allows color to 

 appear in the eyes. The white eyed fly 

 (first described by Morgan '10) is now 

 known to be always recessive with re- 

 spect both to C and to the next factor. 



69 



O. Flies recessive with respect to 

 0(o) have eosin eyes. The relation 

 between C and O has been explained 

 by Morgan in a paper now in print 

 and about to appear in the Proceedings 

 of the Academy of Natural Sciences 

 in Philadelphia. 



P. Flies with p have vermilion eyes 

 instead of the ordinary red (Morgan 

 'II d). 



R. This and the next factor both 

 affect the wings. The normal wing is 

 RM. The vM wing is known as minia- 

 ture, the Rm as rudimentary, and the 

 rm as rudimentary-miniature. This 

 factor R is the one designated L by 

 Morgan ('11 d) and Morgan and Cat- 

 tell ('12). The L of Morgan's earlier 

 paper ('11) was the next factor. 



M. This has been discussed above, 

 under R. The miniature and rudimen- 

 tary wings are described by Morgan 

 Cl'la). 



The relative position of these factors 



is B,-^, P, R, M. C and O are placed at 



the same point because they are com- 

 pletely linked. Thousands of flies had 

 been raised from the cross CO (red) 

 by CO (white) before it was known 

 that there were two factors concerned. 

 The discovery was finally made be- 

 cause of a mutation and not through 

 any crossing over. It is obvious, then, 

 that unless coupling strength be vari- 

 able, the same gametic ratio must be 

 obtained whether, in connection with 

 other allelomorphic pairs, one uses CO 

 (red) as against co (white), Co 

 (eosin) against co (white), or CO 

 (red) against Co (eosin) (the cO com- 

 bination is not known). 



METHOD OF CALCULATING STRENGTH 

 OF ASSOCIATION 



In oder to illustrate the method used 

 for calculating the gametic ratio I 

 shall use the factors P and M. The 

 cross used in this case was, long 



