252 PHYSICAL BASIS OF HEEEDITY 



the case (just stated) for Indian corn and for the fruit 

 fly. If each member of such a series of allelomorphs has 

 arisen historically from the preceding one in the series, 

 by a mutation in a locus closely associated with the locus 

 responsible for the first, they would be expected to give 

 the wild type when crossed; and as the proof of their 

 allelomorphism turns on the failure of members of the 



B 



Fig. 108. — Diagram illustrating mutation in a nest of genes so closely linked that no 



crossing over£takes place. 



series to show the atavistic behavior on crossing, it is 

 necessary, as stated, to know how they arose. This may 

 be made clear by the following illustration : 



Let the five circles of Fig. 108, A represent a nest of 

 closely linked genes. If a recessive mutation occurs in 

 the first one (line B, a) and another in the second gene 

 (line B, h), the two mutants a and b if crossed should give 

 the atavistic type, since a brings in the normal allelo- 

 morph (B) of h, and h that (A) of a. If a third mutation 

 should occur in the third gene it, too, will give the atavistic 



