458 WALTER W. MARSHALL AND HERMANN J. MULLER 



The balloon factor was kept heterozygous for at least fifty 

 generations, covering a period of nearly three years. Hetero- 

 zygous males having the mutant factors streak, ^ dachs, jaunty, 

 curved, and balloon in one chromosome, and the mutants black, 

 purple, vestigial, arc, and speck in the other, were repeatedly 

 crossed to homozygous females containing black, purple, vestigial, 

 arc, and speck. All these mutant factors are recessive except 

 streak, so that the male offspring containing the first namekl 

 combination of factors were easily picked out. Since there is 

 no crossing over in the male and the females used were pure for 

 b-p-v-a-sp, the factors of each kind of chromosome were kept 

 together, and there were only two kinds of males and females 

 produced in each generation, of the same types as those above 

 described. One kind showed the character streak, a marking 

 on the dorsal side of the thorax, and in the other kind this 

 character was absent but the characters oi h p v a and sp were 

 manifested. The males containing streak thus always contained 

 the factors for balloon wing and for its normal allelomorph. 

 The close association of these allelomorphs over such an ex- 

 tended period furnished an excellent opportunity for contami- 

 nation if contamination really occurs. 



Before the balbon character could be studied it was neciessaipy 

 to separate it from the other mutants and obtain flies homozy- 

 gous for balloon. This process of purification was a somewhat 

 complicated one. The accompanying diagram will serve to illus- 

 trate the method of 'freeing' the balloon factor. Males of the 

 heterozygous type just described were crossed with normal fe- 

 males. The Fi offspring were heterozygous, some containing the 

 S dj cha chromosome and the normal chromosome, others con- 

 taining the bpv asp and normal chromosomes. The latter were 

 eliminated. Males and females of the former type were then 

 crossed with each other. Part of the offspring (F2) from this 

 cross received from their mother balloon but none of the other 

 mutant factors (the balloon having crossed over), and from their 

 father they received the chromosome containing Sdj cha. These 



1 The factor streak was introduced in the way described only during the later 

 generations. 



