GENES, MOLECULES AND PROCESSES 



new method wc have additional evidence that the elements of the 

 cuchroniatin are producing complex proteins and therefore have a 

 complex and delicately adjusted organization. In such a system 

 undirected change is likely to break down the old rather than to 

 build up the new. 



Another way of getting at this problem is by examining what 

 appears to be the closest to the gene of all the products of its activity. 

 Antigen production is the most specific and unconditional activity of 

 major genes. The chain of reactions from the gene in the nucleus 

 to the antigen in the cell is therefore the shortest we can infer. All 

 organisms contain antigens since any protein can behave as an 

 antigen. But in the blood groups which govern the possibilities of 

 transfusion in man and other animals, different antigens are directly 

 associated with corresponding genes. If the gene is present so is the 

 antigen. 



There are many sets of blood-group alleloniorphs such as M-N, 

 and the Rliesus series. In a heterozygote of any of them the antigens 

 of both allelomorphs are present in the cell — notably, of course, in 

 the red blood cells where they are recognizable if the appropriate 

 antibody is induced. In the important A-B-O series in man and the 

 apes, it happens, for a reason that is not yet understood, that the 

 antibodies to A and B occur spontaneously, whereas O cannot even 

 induce one easily. Thus O is recessive to A and B and serologically 

 appears merely as their absence (Fig. 37). On the other hand, in 

 certain species hybrids of doves new antigens appear to arise which 

 are found in neither parent; thus the co-operation of more than 

 one gene in their production is implied. Otherwise, so close is 

 the coimexion, that the distinction between gene and antigen 

 is formally maintained only because of the necessity for disting- 

 uishing between determinant and product which genetics has 

 taught us to respect. Now the blood antigens, and likewise the 

 antibodies they call forth, are complex proteins; we therefore have 

 the best evidence that such proteins can be the immediate products 

 of gene activity. 



The Interaction of Genes 



The relation between the gene and the antigen it produces seems 

 generally to be a simple one; as simple indeed as that between the 



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