Linkage Group IV. 



43 



V. LINKAGE GROUP IV. 



In group IV only three mutant characters are known, acute, 

 pinched, and hump. The first is sometimes difficult to classify, but 

 the other two are excellent. Their linkage relations may be sum- 

 marized by saying that up to the present time no crossing-over has 

 been detected between any two of the three, although fairly large 

 counts have been obtained. 



Table 5. — Summary of cross-over experiments in group III. 



Experiment 

 No. 



Mating. 



Cross-over percentage. 



Total 

 flies. 



60 

 57 

 58 

 56 

 61 

 59 



S X h t 



S Xh 



S Xt 



S X 6p 



S h t X G. . 

 sp X G 



Counting all: S-h 40.8, h-t 18.8 j 1,137 



Counting only S: S-h 31.3. 



Counting all: S-h 26.1 



Counting only S: S-h 14.0 



Counting all: S-t 43.0 



Counting only S: S-t 39.6 



Counting all: S-sp 23.8 



Counting only S: S-sp 21.1 



Counting axl: S-h 32.8, h-t 9.1, t-G 42.9. 



Counting only S: S-h 30.9 



Counting all: sp-G 41.8 



479 

 134 



64 

 650 

 240 

 227 

 109 

 219 



84 

 141 



Table 6. — Summary of cross-over data on group III, arranged according to regions. 



Region. 



Exp. No. 



Total flies. 



Cross-overs. 



Per cent. 



S-sp 

 S-h 



h-t 



t-G 

 ep-G 



56 

 56 

 60 

 60 

 57 

 57 

 61 

 60 

 61 

 61 

 59 



227 

 109 



1,137 



479 



134 



64 



219 



1,137 

 219 

 219 

 141 



, counting 

 counting 



, counting 

 counting 

 counting 

 counting 

 counting 



all flies. . . . 

 only S flies . 

 all flies. . . . 

 only S flies, 

 all flies. . . . 

 only S flies, 

 all flies. . . . 



54 



23 



465 



150 



35 



9 



72 



214 



20 



94 



59 



23.8 

 21.1 

 40.8 

 31.3 

 26.1 

 14.0 

 32.8 

 18.8 

 9.1 

 42.9 

 41.8 



DESCRIPTION AND ORIGIN OF CHARACTERS IN GROUP IV. 



Acute (ac). (Plate 3, Figure 10.) 



Description. — Acute is characterized by short, pointed wings and small anterior 

 dorso-central bristles. The posterior and often the anterior sterno-pleural bristles 

 are reduced to hairs, the anterior and posterior scutellar bristles are close together, 

 and one or more of the orbital bristles are frequently absent. In addition the fifth 

 vein, and less often the second vein, is slightly shortened. The flies tend to be small 

 and to have short legs, but they can not be classified accurately on this basis. 



Origin. — (See Metz, 1916, p. 597.) Acute arose in concave stock and was at first 

 thought to be a form of concave. 



