Mutation 113 



to this have been noted. Muller (9) cites a case in which the 

 different mutant genes at the same locus may cause either shorten- 

 ing of wing, eruption on thorax, or a lethal effect. 



At the same time, although we are thus repeatedly encounter- 

 ing evidence on the discontinuity of mutation, it is possible that 

 there is an underlying continuity of a sort that we are not in a 

 position to measure. A statement of JVIorgan's (6) bears on this 

 point. "Evidence is fast accumulating that common genes 

 probably undergo analogous mutation in related species, the 

 direction being conditioned by the physico-chemical constitution 

 of the gene and not by some hypothetical directive force." 



2. CoMPLEXMUTATiONS. — It is perhaps surprising that, in 

 spite of the numerous cases of locus changes that were being 

 discovered, there were for a long time no clear cases of mutations 

 involving simultaneous changes in several neighboring factors in 

 one region of a chromosome. Nilsson-Ehle (ii) now claims to 

 have such a case, and calls it ''complexmutation." Normal wheat 

 mutates to bearded speltoid, involving a simultaneous change 

 in two closely linked genes. Among the F2 progeny of normal A' 

 mutant appear a few bearded normal type and beardless speltoid, 

 but only a few, due to the very close linkage of the two mutated 

 genes. In another case the same investigator claims that three 

 linked factors have mutated simultaneously. 



3. Deficiency. — A rare phenomenon has been described by 

 Bridges (3), working on the fruit fly. "Deficiency" as he calls 

 it, is something more extensive than a simple locus change (and 

 probably more extensive than the complexmutations). It is a 

 "regional mutation," involving an "inactivation" of a portion of 

 a chromosome, so that the genes on that region of the chromosome 

 are rendered ineffective (nor can crossing over take place in that 

 region). 



ii. chromosome changes ('' chromosome 

 aberrations") 



I. Duplication.— Bridges (3) describes another rare type 

 of change in the germ plasm, to which he gives the name "duplica- 

 tion." Judging from the very unusual breeding results obtained, 

 some abnormality in connection with mitosis has resulted in the 



