170 A. R. Moore 



Therefore, accepting the presence and absence hypothesis and 

 adopting a biochemical theory of inheritance, we may say that each 

 germ cell from the dominant parent contains a full amount of the 

 enzyme underlying the development of the dominant characters while 

 the germ cells of the recessive contain none. The hybrid resulting 

 from the cross between these two parents would have but one half 

 the normal amount of enzyme necessary for the development of the 

 dominant characters. Now the velocity of a chemical reaction de- 

 pends upon the concentration of the reacting substances and in enzyme 

 reactions varies directly as the concentration or as the square root 

 of the concentration of the enzyme. The final equilibrium in enzyme 

 reactions in most cases is unaffected, in other cases it bears some 

 relation to the concentration of the enzyme *). 



If these laws are operative in the development of organisms, 

 then in our hypothetical F^ individual, the dominant character should 

 appear more slowly than in the pure dominant strain, although final 

 equilibrium may be the same. The perfect development of dominant 

 characters may be inhibited 1) by the cessation of reactions upon 

 the products of which such characters depend for their full develop- 

 ment. Thus the vegetative growth of many plants ceases when 

 sexual products are formed. 2) The equilibrium of the reaction may 

 be affected by the concentration of the enzyme. In either of these 

 two cases the development of the dominant character will not be 

 complete and as a result we shall have only intermediate domi- 

 nance. 



Now the rate of development of a dominant character in the 

 heterozygote may be retarded but the process finally reaches perfect 

 equilibrium so that the heterozygote cannot be distinguished from 

 the pure dominant. Perhaps the best illustration is in the case of 

 LANG'S 2 ) experiment with snails. In these experiments LANG crossed 

 red and yellow parents. In the F 2 generation, when the animals 

 were young, the heterozygotes were yellow so that the ratio appeared 

 3 yellow: 1 red. As they grew older the red pigment developed in 

 the heterozygotes and at maturity the ratio was 3 red: 1 yellow 3 ). 



1) TAYLOR, A. E., Journ. Biol. Chem. (1910.) Vol. 8. p. 503. 



2 ) LANG, tlber die Bastarde von Helix hortensis Miiller und Helix nemo- 

 ralis L. Eine Untersuchung zur experimentellen Vererbungslehre. (1908.) Jena. 



3 ) SHULL uses this example to explain the apparent dominance of absence 

 over presence and concludes that in such cases the positive character is only 

 latent in the heterozygote. Am. Naturalist. (1909.) Vol. 43. p. 410. 



