82 PHYSICAL BASIS OF HEREDITY 



normal allelomorphic characters, wild-type body color and 

 long wings, have also come out together. There are no F2 

 flies that are black and long, and none that are vestigial 

 and gray, as would be the case if independent assortment 

 took place. 



In the diagram (Fig. 33) the results are worked out on 

 the chromosome theory. The genes for black (5) and for 

 vestigial {v) are represented as carried by the same 

 chromosome {hv) -, the homologous chromosome of the 

 wild-type fly carries the normal allelomorphic genes {BV). 

 In Fi, one of each of these two chromosomes is present, 

 and the fly is normal because the two normal allelomorphs 

 are dominant. In the F^ male these two chromosomes 

 {hv and BV) separate at the reduction division of the 

 germ-cells, one going to each gamete. If this F^ male is 

 mated to a black vestigial female, all of whose eggs carry 

 genes for black and for vestigial, the offspring should 

 reveal the composition of the gametes of the F^ male, since 

 the eggs of the black vestigial fly, containing only two 

 recessive factors, will not cover up the effects of the fac- 

 tors contained in the gametes of the F-^ male. 



Unless we knew that the two characters black and 

 vestigial are distinct mutant characters, the preceding 

 experiment would not necessarily show that the char- 

 acters are linked, because the same result would have 

 followed if black and vestigial were both due to the effect 

 of a single gene. Other experiments, however, show that 

 they are independent characters. 



It is interesting to compare the preceding cross with 

 another in which black comes in from one parent, and ves- 

 tigial from the other. For instance, if a black fly with 

 long wings is crossed to a wild-type fly with vestigial wings 

 (Fig. 34), the F^ offspring will be wild type both in their 

 color and in their wings, because the black fly brings in 

 the normal allelomorph of vestigial, and the vestigial fly 

 brings in the normal allelomorph of black. If the F^ 

 sons are back-crossed to black vestigial females, the off- 



