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ters, black body color and vestigial wings (Fig. 33), is 

 mated to a fly with wild-type body color and long wings, 

 the offspring {f\) are wild type. If one of the F^ sons 

 is back-crossed to a black vestigial female from stock, 



Fig. 33. — Back-cross o£ Fi male (out of black vestigial by wild), to black vestigial. 



the offspring {F^) are of two kinds only, half are black 

 vestigial, and the other half are wild type. In other 

 words, the two mutant characters that went in together, 

 black and vestigial, have come out together; and their two 

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