250 Mechanism of Mendelian Heredity 



between the localization of a factor in the chromosome 

 and the production of a comparatively large quantity of 

 a specific chemical compound, e. g., a chromogen or a 

 tyrosinase. We must remember that all the cells of 

 an organism have identical chromosomes, so that 

 a factor for an enzyme like tyrosinase is contained 

 in every cell throughout the whole body. It is 

 likely, however, that the same factor (which we may 

 conceive to be a definite chemical compound) will 

 find a different chemical substrate to work on in 

 the cells of different organs of the body, since the 

 different organs differ in their chemical composition. 

 Thus it is conceivable that in the production of tyro- 

 sinase or of tyrosine not a single chromomere of one 

 single cell is engaged, but the sum total of all these 

 individual chromomeres of all the cells in one or several 

 organs of the body. The writer has added this remark 

 especially in consideration of the fact that some authors 

 seem to feel that the chromosome conception of heredity 

 is incompatible with a physicochemical view of this 

 process. 



Since we have mentioned this difficulty which some 

 writers seem to find in the chromosome theory of Men- 

 delian heredity, it may be added that a single factor 

 may suffice to determine a series of complicated reflexes. 

 Thus the heliotropic reactions of animals are due to the 

 presence of photosensitive substances, and it suffices 

 for the hereditary transmission of the complicated 



