THE INHERITANCE OF BIOCHEMICAL CHARACTERISTICS 305 



The carotenoid concentration increases in the following order of 

 genotypes 



rT rt Rt RT 



and it can be seen that the chief result of substituting R for r is an increase 

 in the amount of carotenoid. 



The flower pigments, which we described under the name anthocyanins 

 and whose glycones are called anthocyanidins, have been the object 

 of extensive genetic studies carried out at the John Innes Horticultural 

 Institute. The anthoxanthins by their presence can modify the colour of a 

 flower determined by the anthocyanins. A change in a single gene may 

 markedly alter the colour of a flower. For example, in the snapdragon 

 the yellow or white colour depends on particular combinations of the 

 same pair of allelomorphs, Y and y. A yellow colour corresponds to the 

 genot}'pes YY or Yy, whilst the recessive plants yy have white flowers. 

 The reader will find a number of descriptions of biochemical characteristics 

 controlled by genes in the work of Wagner and Mitchell (1955). 



11. BIOCHEMICAL DIFFERENTIATION OF CELLS 

 IN A SINGLE ORGANISM 



In the course of the ontogenesis of a multicellular organism, the various 

 cells, although characterized by having the same genot}^pe, may be different 

 biochemically. The process called "determination" by the embryologists 

 causes one or other set of genes to act, the result being apparent in the 

 differentiation of each class of cell, resulting from the influence of the 

 cytoplasm on the activity of the genes or on the products of genes. The 

 biochemical diversity of organisms is hence in fact a diversit}^ of cell 

 aggregates which are themselves biochemically differentiated. 



The explanation of how cells coming from the same zygote are differen- 

 tiated biochemically, belongs to the domain of chemical embryology and 

 chemical genetics and is still largely conjectural. 



In a remarkable series of studies on the content and history of the cell 

 theory, John R. Baker (1948) has provided a modern formulation of the 

 theory in the following seven propositions : 



"I. Most organisms consist of a large number of microscopic bodies 

 called 'cells', which, in the less differentiated tissues, tend to be 

 polyhedral or nearly spherical. 



"II. Cells have certain definable characters. These characters show that 

 cells (a) are all of essentially the same nature and (b) are units of 

 structure. 



"III. Cells always arise, directly or indirectly, from pre-existing cells, 

 usually by binary fission. 



