62 INBEEEDING AND OUTBREEDING 



a cross in a group and be segregated in a group in the 

 F2 generation. 



Such cases have appeared in breeding experiments, 

 but they are very rare and are probably not what they 

 seem, because of an insufficient number of individuals 

 from which to draw conclusions. What usually happens 

 is for these sets of factors to tend to hang together at the 

 reduction division in the F-^ generation. They tend to be 

 linked, but the linkage is often broken. 



An example from Morgan's work on the pomace fly 

 will make this clear. If a female fly with black body and 

 vestigial wings be crossed with a wild male having a gray 

 body and long wings, the result is offspring like the male, 

 gray body and long wings being dominant. Now, since 

 these Fj individuals have one chromosome containing the 

 factors for black body and for vestigial wings, and a 

 homologous chromosome containing the factors for gray 

 body and for long wings, one would expect gametes of 

 only two kinds to be formed at the maturation of the eggs 

 and sperms. If this were true, an F2 generation ob- 

 tained by mating a male and a female from the F^ genera- 

 tion should consist of 3 flies having long wings and 

 gray bodies to 1 fly having vestigial wings and a black 

 body. But this is not the result obtained. In addition to 

 large numbers, of flies of this type, there are smaller num- 

 bers of flies characterized by long wings and black bodies, 

 and by vestigial wings and gray bodies. Such a result, 

 on a chromosome basis, could only be obtained through the 

 homologous chromosomes interchanging their factors at 

 the reduction division. 



There is good cytological evidence that such an inter- 

 change of chromosome parts does take place at game- 



