Linkage Data. 45 



Pinched and hump. — The linkage of pinched and hump was verified 

 by both F2 counts and back-crosses. In the latter, males heter- 

 ozygous for pinched and hump, in the same chromosome, were 

 back-crossed to hump females and gave (L 338, L 344) : 99 wild-type, 

 77 pinched hump, no pinched, and no hump offspring. 



Complete Linkage of Acute, Pinched, and Hump. 



Further matings involving acute, pinched, and hump have 

 indicated an extremely close, if not complete, linkage between them. 

 Up to the present time no certain case of crossing-over has been 

 obtained among 151 flies in back-crosses involving pinched and 

 hump (experiment 62) and 978 flies involving acute and pinched 

 (experiment 63), where heterozygous females were used and cross- 

 overs would be expected to appear. In one culture a single fly with 

 abnormal wing venation was found which was recorded as possibly 

 pinched, and may have represented a cross-over. Unfortunately 

 it died without giving any progeny. 



The dissimilarity of the three characters involved makes it im- 

 probable that they are allelomorphs, as does also the fact that no 

 effect has been observed on the heterozygotes involving hump and 

 acute, or pinched, hump, and acute. It should be noted that these 

 flies were not examined specifically with this point in mind, but it is 

 unlikely that any appreciable effect of either gene would have been 

 overlooked. Since pinched appeared in connection with hump, 

 and no cross-overs have been detected, the gene for pinched has 

 always been accompanied by that for hump, and it is not known 

 whether pinched would be altered if the gene for hump were elim- 

 inated. 



The complete linkage involved here suggests at once that the genes 

 might be in the small spherical m-chromosome that resembles the 

 "fourth" chromosome of D. melanog aster, in which there appears 

 to be very little crossing-over (see p. 76). 



