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 2 Normal o 
Py (B’X)(B'X) x (W/X)Y 
Gametes (B’X) a ae (b/X) 
a 
Fy (B'X) (x) “~(B'X) Y 
Bar-eyed 92 Bar-eyed 
Fic. 70.—Results of mating bar-eyed 2 with normal-eyed o 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. 
Fic. 71.—Longitudinal sections of corn grains showing differences in character of starch; 
left, foury; 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. 
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