144 THE SECOND-CHROMOSOME GROUP 



glance, because normally the males show a considerably smaller per- 

 centage of truncate than do the females. The difference in culture 

 II was especially striking. 



A recent census of the truncate stock, Table 14. 



which has been run for about 5 years under 

 selection not especially rigorous, showed 

 about 17 per cent of wild-type flies (table 14). 

 The truncate flies were of various degrees of 

 shortness, of which the most frequent was 

 that known as "intermediate" (correspond- 

 ing to fig. 4 of Plate 6). The very short 



truncates were not especially numerous, although in selecting the 

 parents each generation they had been preferred. Table 14 gives the 

 census of truncate stock (May 1917). 



BLACK (h). 



(Plate 5, figure 2.) 



ORIGIN OF BLACK. 



The first workable body-color mutation, black, was found by Morgan, 

 October 1910, in the F2 of a cross between miniature and wild flies 

 (Morgan, 1911). 



DESCRIPTION OF BLACK. 



When black flies are freshly hatched little black color has developed 

 on the body, though the legs and feet are darker than normal. Within 

 a few minutes after hatching the color has deepened so that the head, 

 thorax, and abdomen are a clean, fresh, greenish black, more intense 

 on the thorax than on other parts. This color becomes progressively 

 darker with age. The wings, after expanding, also become much 

 darker, and along each side of the veins a broad band of pigment begins 

 to develop and becomes conspicuous in old flies. 



SEMI-DOMINANCE OF BLACK. 



While the fly heterozygous for black is noticeably darker than the 

 wild-type, this separation can not be made completely, although it is 

 occasionally made use of for special purposes (see sections on Jaunty, 

 p. 162, and Apterous, p. 237). Black is generally, therefore, treated 

 as a recessive, and the separation of black from the heterozygote is 

 easy and entirely accurate. 



CHROMOSOME CARRYING BLACK. 



Black is a member of the second-chromosome group by definition. 

 As soon as it had been established that the loci for the sex-linked 

 mutations were capable of being mapped in definite positions (Sturte- 



