502 THE AMERICAN NATURALIST [Vol. XL VIII 



THE THEORETICAL DISTINCTION BETWEEN 

 MULTIPLE ALLELOMORPHS AND 

 CLOSE LINKAGE 



Professor Castle 's difficulty in understanding the distinction 

 made by Mr. Dexter 1 is owing to his unfamiliarity at first hand 

 with the phenomenon of linkage. The distinction between allelo- 

 morphs and close linkage has already been given several times 

 elsewhere and need not be repeated; but if Professor Castle has 

 failed to note it, or to see its significance, it is probable that 

 others may have done the same. I may be pardoned, therefore, 

 for attempting once more to show why, for clear thinking, it is 

 important to keep in mind the difference between allelomorphs 

 and close linkage. Furthermore, since we have here one of the 

 newest developments of Mendelism, it seems to me that it may be 

 worth while not to let Professor Castle's criticism pass un- 

 challenged. 



Dexter pointed out that the mode of treatment that Nabours 

 followed in the analysis of his results is the procedure of multiple 

 allelomorphism, although Nabours does not seem entirely con- 

 versant with the fact, but treats the results as though they were 

 regular phenomena. In one case, however, Nabours got an un- 

 conformable individual. Dexter points out that if this case is 

 not due to non-disjunction (a known process that will cover 

 such cases) it shows that here at least the factors involved are 

 not allelomorphs, but must be treated as though closely linked. 



How could the matter be put more directly? I confess I am 

 at somewhat of a loss to discover why Professor Castle is con- 

 fused. Perhaps it is the subsequent development of Dexter 's 

 explanation that has troubled him. Let us again try to make the 

 distinction clear. 



If the factors B and E are not allelomorphic to each other then 

 each must have another allelomorph. This is nothing but pure 

 Mendelism, which no one will, I suppose, dispute. It is entirely 

 irrelevant whether we use small letters or none at all (as Castle 

 prefers) for the allelomorphs. If they are a part of the Men- 

 delian machinery, who cares very much what we call them? 



If then we have here two pairs of allelomorphs, crossing over 

 may take place, as it does in other cases where two pairs of linked 



June, 1914, p. 383. 



