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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 wiJd type. If one of the F, sons 

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



p/ /- 



Fig. 33. — Back-cross of Fi male (out of black vestigial bj' wild), to black vestigial. 



the offspring (F.) are of two kinds only, half are black 

 v^estigial, and the otiier half are wild type. In other 

 words, the tw^o mutant characters that went in together, 

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

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