OF MUTANT CHARACTERS. 239 



wild-type brothers heterozygous for apterous succeeded (table 87). 

 There were produced a total of 358 flies, of which only 43 or 12.0 per 

 cent were apterous. Of these 43 the star apterous cross-overs num- 

 bered 13 and the simple apterous non-cross-overs 30. 



These flies are comparable in viability and the percentage of cross- 

 ing over can be calculated directly from them as 30.2. Another 

 calculation can be made from the not-apterous flies, of which there 

 were 155 stars and 160 wild-type. Both these classes are composite, 

 the stars comprising two non-cross-over and one cross-over class 

 (2n-}-x = 155), while the wild- type flies comprise one non-cross-over and 

 two cross-over classes (n+2o; = 160). From these equations n = 50 

 x 55, or the percentage of crossing over is 52.3. 



/S' X\ 

 One cross of star female by star male both ( ) produced 



\ /\ dp/ 



offspring only one of whose classes is composite (table 88) . As before, 

 the star apterous flies are cross-overs (2 = 9) and the apterous non- 

 cross-overs (n = 7) which gives 9 cross-overs out of 16 or 56 per cent. 

 The wild-type class is a simple non-cross-over (n = 6) to be compared 

 with the complex class star (2n-fx = 24). From these equations 

 x = 6 and n = 9, or the percentage of crossing over is 40.0. The total 

 amount of crossing-over data derived above gives the equivalent of 

 169 flies, of which 83 or 49.0 per cent are cross-overs. From a com- 

 parison of this value (49.0) with the cross-over values given by star 

 and other second-chromosome genes, the locus of apterous is found to 

 be most probably somewhat to the right of black. This locus agrees 

 perfectly with the position found by Metz on the basis of the black- 

 apterous F 2 described above. A position at 48.5 is indicated as approxi- 

 mately correct. 



Cm AND Cn r . 



A paper by Sturtevant, giving a full account, with summaries, of the 

 work done on the two second-chromosome cross-over variations Cn, 

 and Cn r appears herewith (Part III), to which the reader is referred. 



CREAM II (Cr,,). 1 



(Plate 5, figure 11.) 



ORIGIN AND DESCRIPTION OF CREAM II. 



A pure stock of the sex-linked eye-color eosin shows a strong sexual 

 bicolorism; that is, the eye-color of the eosin female is a rather deep 

 pink only slightly yellowish, while the eye-color of the eosin male is a 

 pinkish yellow much lighter in tone than the color of the female (see 

 Morgan, 1912, for an account of the origin of eosin and a colored plate 

 showing this difference in color). Eosin females and males maintain 



1 A short reference to the case of cream II, and a discussion of its bearing on the question of 

 multiple factors was made in the "Mechanism of Mendelian Heredity," p. 203. 



