150 



GENETICS IN RELATION TO AGRICULTURE 



the same genetic constitution, therefore, as a male of a pure bar-eyed 

 race, and it is to be expected that he will display the character to the 

 same extent as a male from a pure race. On the other hand the female 

 has one X-chromosome which bears the normal recessive allelomorph 

 of the bar-eye factor. This factor may be considered as exerting a 

 competitive influence against the bar-eye factor of the other X-chromo- 



Bar-eyed 9 

 P, (B'X)(B'X) 



Gametea 



(B'X) 



fD'V\ ff\'V\^ ~~~^^(D'Y\ V 



(HA.) {DJt) (tSJi) I 



Bar-eyed 9 Bar-eyed cf 



Fio. 70. Results of mating bar-eyed 9 with normal-eyed cf Drosophila. 



some, so that the character expression in a sense depends upon a variable 

 equilibrium reached between the two factors. Since they appear to be 

 nearly equal in potency it is possible apparently for this equilibrium to 

 be thrown so much to one side or the other that at times the character 

 expression approaches that of the typical bar-eyed strains and at times 

 that of the normal round-eyed flies. 



FIG. 71. Longitudinal sections of corn grains showing differences in character of starch; 



left, floury; right, flinty, 



An interesting case which throws considerable light on the competi- 

 tive action of factors in determining character expression has been 

 reported by Hayes and East in maize. Flint races of maize are char- 

 acterized by the production of a very small amount of soft starch in 

 the center of the seed and a large amount of hard corneous starch sur- 

 rounding it. Flour corns on the other hand produce grains the endo- 

 sperm of which is almost wholly made up of soft starch with occasionally 

 a very thin layer of corneous starch at the exterior of the endosperm. 

 These differences are shown in Fig. 71. 



