THE GROWTH OF GENES 



predominant importance in determining the agglutination of the 

 blood in late embryos of D type. The mode of action is that, after 

 the mating of a D father with a d mother, a D embryo "immunizes" 

 its mother against D by causing her to produce the A antibody 

 and so jeopardizes for many years to come any D embryo that 

 may follow it. The other antigens act less often because they are 

 less efficient immunizcrs. 



Thus the Rliesus gene can be looked upon as built up of three 

 genes generally transmitted as a unit but always independent of 

 one another in action. Each of these three can exist in two common 

 forms : C — c D — d and E — e. Each of the forms yields a single 

 antigen and hence can induce a single antibody. Thus there should 

 be eight combinations (seven of which are now known) each 

 yielding three antigens and so reacting with three sera. Equally 

 there should be six sera (five of which are now known) each 

 reacting with four combinations or, from the point of view of 

 the Rhesus gene as a whole, four allelomorphs. 



We have said that the three parts of Rhesus are independent in 

 action. But, on the basis of what we know of the S gene, we may 

 well suppose that they have a unity of action based on a carrier, 

 and that the allelomorphs of C, D and E, are merely stamping 

 products of this common carrier with specificity. It is indeed already 

 certain that the stamps used are different in man and the cliimpanzee. 

 On this view the number of allelomorphs will be limited only by 

 the number of specifying groups that can attach themselves to the 

 carrier. 



The Rhesus and incompatibihty genes agree in showing us natural 

 integration. They also agree in showing us, what we could by no 

 means have expected, that the carrier and the key have to travel 

 next to one another on the chromosome. Co-operation requires 

 juxtaposition, and the two underlie integration. 



Tying Genes Together 



These observations clarify the growth of genes as units of action. 

 Such a development presupposes their corresponding growth as 

 units of transmission. But the converse, as we saw earher, is not 

 necessarily true. Holding together a group of genes will in the first 

 instance give us, not a more integrated unit, but a multifarious or 



334 



