252 



PHYSICAL BASIS OF HEREDITY 



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 h if crossed should give 

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

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

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



