30 TYPES OF MENDELIAN HEREDITY 



in others the eye is nearly as narrow a bar as that of 

 pure stock. In the male, which has one factor for 

 bar eye, the eye is as narrow as in the pure (i.e., 

 homozygous) female with two factors. The inter- 

 mediate condition in the female which is hybrid 

 (heterozygous) for this factor is hence not explained 

 by the lesser effect of the single factor, but is probably 

 due to the competing influence of the other allelo- 

 morph. Of course it might be contended that since 

 in the male there is a different chromosome complex 

 (XABCD YABCD) from that in the female (XABCD- 

 XABCD) it is this difference in other factors that 

 causes the heterozygous female to have a wider eye 

 than the male; but this argument is rendered improb- 

 able here, when we recall that in only one out of many 

 cases of sex linked inheritance, in which the hetero- 

 zygous female is intermediate, is the male different 

 from the homozygous female. 



In other cases the influence of one of the parents 

 of the cross may be so slight as to escape detection 

 on ordinary observation, and may require special 

 measurements for demonstration. When flies with 

 miniature wings (Fig. 16) are mated to wild flies, 

 the daughters have long wings, which Lutz has shown 

 to be a little shorter in proportion to the length of 

 the legs than are the wings of wild females; but the 

 difference is so slight that it could not have been 

 detected without biometrical methods. 



Finally, we must consider the class of cases in 

 which complete dominance has been described. All 

 the cases given by Mendel in peas were supposed 



