No. 576] 



VARIATION IN DROSOPIIILA 



721 



and normal F x flies should give the same results when 

 used as parents. Or if we were dealing here with a case 

 like the "yellow mouse" case, in which homozygous 

 yellows do not exist: that is, if homozygous "Beadeds" 

 do not exist, then one quarter of the flies produced by two 

 Beaded parents from the stock should be normal. But as 

 was said before, the stock breeds true, every fly produced 

 having Beaded wings. 



It may be noted that a pair of Fj normal flies usually 

 produce less than 10 per cent, of Beaded offspring. If 

 these normal flies carried a recessive gene for Beaded- 

 ness, they should produce twenty-five per cent. Beaded 

 offspring. The.Beaded Fj offspring, on the other hand, 

 though they produced in all cases more than twenty-five 

 per cent., did not produce 75 per cent. Beaded offspring, 

 as they should have done if a single dominant gene for 

 Beaded wings were heterozygous in them. 



3. Behavior in Third and Fourth Generations 

 Beaded offspring, that appeared in the P, generation 

 of the cross Beaded X Wild, were back crossed to Wild. 

 The process was again repeated with the Beaded off- 

 spring that appeared, till four generations had been pro- 

 duced. The results of this test are given in Tables VII 

 and VIII and in Chart 4. 



A striking result is that an F 1 Beaded fly or even a fly 

 of later generations heterozygous for Beaded wings some- 



table vir 



Repeated Back-crosses of Beaded- winged Flies from the Cross Beaded 



