216 MULTIPLE ALLELOMORPHS 



or part of the body that is affected. It seems there- 

 fore more probable that this pecuHar fact connected 

 with multiple allelomorphs means that the same 

 portion of the chromosome is changed in one or 

 another direction. 



3. It is true that a very wide range of linkage values 

 has been obtained, that extends from almost free 

 segregation to less than 1 per cent, of crossovers. 

 However, if we should construct a curve showing the 

 number of cases exhibiting the various possible 

 linkage values, the number showing complete linkage 

 or, as we should say, multiple allelomorphism, would 

 be far in excess of the number of these to be expected 

 from the general shape of the rest of the curve. This 

 indicates that multiple allelomorphs are in a class 

 by themselves, not merely extreme cases of the same 

 type as an ordinary linkage case. 



4. There is an a priori consideration that may not 

 be out of place in the argument. There is no suffi- 

 cient reason for supposing that only one sort of 

 mutation can occur in a given locus in the chromo- 

 some. If the basis of the chromosome is a chain of 

 chemically complex substances {e.g., proteins), any 

 slight addition or loss or even re-arrangement of the 

 atoms in the molecules of a bead in such a chain 

 might well produce an effect on the organism, and 

 perhaps a more marked effect on that particular 

 character that stands in closest relation to that 

 chemical body. Since we know that mutations and 

 even "reverse" mutations actually occur, it would be 

 indeed strange if only one kind of change were 



