OF MUTANT CHARACTERS. 149 



character is the presence of extra veins in the wings, which should be 

 looked for in checking up whether any balloon flies with only slight 

 divergence have been passed over in the preliminary separation. 

 These extra veins are most plentiful as a plexus about the posterior 

 cross-vein and also between the marginal and second longitudinal 

 veins (plate 7, fig. 1) . The balloon wing is usually considerably smaller 

 than normal and is of a brownish uneven color and of markedly chiti- 

 nous appearance. The balloon flies run about very actively and take 

 short, quick jumps, but are unable to fly. 



Since balloon appeared in truncate stock it partook of the sterility of 

 truncate. By out-crossing and extraction a stock was obtained which 

 was fully fertile and which was free from truncate. 



CHROMOSOME OF BALLOON. 



That the gene for balloon is hi the second chromosome was shown 

 by Sturtevant (April 20, 1912) by means of a cross to curved, which 

 gave in F 2 the 2:1 : 1 : ratio typical in Drosophila for experiments 

 with genes in different homologues of the same chromosome pair. 



LOCUS OF BALLOON. 



Sturtevant also found that the locus of balloon is very far away 

 from that for black (black balloon cross-over value 48.3), and is in fact 

 in the right-hand end hi the same region with speck. Speck and bal- 

 loon were found to be so close together that all attempts to synthesize 

 the double recessive, speck balloon, failed. Without this double 

 recessive it was impossible to run a back-cross test which would have 

 told on which side of speck the balloon gene is situated and exactly 

 how far distant. 



By the laborious method of testing individually the offspring of 



females heterozygous for speck and balloon f J-r- ) Muller (1916) 



found that two 1 individuals out of a total of 462 represented crossing 

 over between speck and balloon. Balloon is therefore about 0.4 unit 

 away from speck, and to the right as was shown by the other second- 

 chromosome characters tested at the same time. 



On account of its marked variability, the character balloon has been 

 used by Marshall and Muller (1917) in a study of the question of the 

 contamination of "a gene by its allelomorph when the two are present 

 in the heterozygote. By back-crossing in each generation a male 

 heterozygous for balloon, and for certain other characters used as 

 indexes, to a female which has these index characters but is free from 

 balloon, a stock was carried on for some 50 generations (nearly 3 years), 



1 These two cross-overs were inadvertently omitted from the table of page 422 (Muller 1916) 

 and from his summary on page 423. 



