ALLELOMORPH1C RELATIONSHIPS IN MEN DELI SM 



149 



When the F z was grown from such an F 2 population Miss Marryat 

 obtained excellent agreement with this analysis as is shown by the data 

 in Table XXIX. 



Variable Character Expression in the Hybrid. Sometimes the 

 character expression in F\ while intermediate displays a range of varia- 

 tion extending almost from one parent to the other. This is shown rather 

 strikingly in the case of bar eyes in Drosophila (Fig. 69) . The bar eye 

 factor is a sex-linked mutant factor which is responsible for the pro- 

 duction of flies with long narrow eyes instead of the round eyes normal 

 for the species. When a female with bar eyes is crossed to a normal 

 male the FI all have bar eyes. In the males especially the eyes are 



FIG. 69. Normal (a, a') and bar eye (b, b') of Drosophila; shown in side view and as seen 

 from above. (After Morgan.) 



just as narrow as in homozygous races, but among the females some 

 may be found which have eyes nearly as narrow as those characteristic 

 of homozygous bar eye flies and others which have eyes nearly as round 

 as those characteristic of the normal fly. Most of them, however, have 

 eyes which display an intermediate effect of the factor. 



This case readily admits of explanation, if the genetic phenomena 

 involved are considered. Since the factor for bar eyes is sex-linked we 

 may represent the bar-eyed female as (B'X)(B'X), folio whig Morgan 

 in employing the primed symbol to indicate a dominant mutant factor. 

 The male with normal eyes is then (b'X) Y. When a bar-eyed female is 

 mated to a normal male, bar-eyed females and males are obtained in 

 FI as shown in the diagram in Fig. 70. 



The FI bar-eyed male obtains his only .X-chromosome from the female 

 and this chromosome contains the factor for bar eyes. He has exactly 



