256 



STADLER 



in the absence of further evidence, 

 necessarily imply that the element is 

 unitary. The genie elements are desig- 

 nated by symbols not italicized, such 

 as P, S, D, and so forth. 



In addition to the crossover mutants 

 there are numerous noncrossover mu- 

 tants. A noncrossover r^ mutant is 

 presumably of constitution "P s" 

 rather than merely "P." The postu- 

 lated element (s) is a "null" element 

 phenotypically but presumably would 

 function synaptically in the same way 

 as "S." These postulated elements are 

 designated "s," "p," "d," and so forth. 



The complex may, of course, in- 

 clude other null elements from past 

 mutations in which the parental ele- 

 ments are unknown. These as a class 

 are designated as "n." 



In several instances noncrossover 

 mutants to intermediate levels of seed- 

 color expression occurred including 

 various dilution and pattern types. 

 These are designated "S''," "S**," and 

 so forth. 



Once any two of these genie ele- 

 ments have become established in 

 neighboring positions in the same chro- 

 mosome, an opportunity is provided 

 for unequal crossing over, which may 

 ultimately lead to the development of 

 more complex gene clusters. For ex- 

 ample, the aforementioned crossover 

 mutants resulted from interchanges as 

 follows: 



P S 



/ 



P s'^P P S 



P s 



PS ^P s s 



(1) 



The crossover-product "S" was rec- 

 ognizable as a crossover mutant R" and 

 the crossover-product "P" as cross- 

 over mutant r''. The crossover-prod- 



ucts "P P S" and "P S S" were not 

 recognizable, but these represented the 

 production of potential new alleles 

 carrying three genie elements instead 

 of two. By using distinguishable forms 

 of S or P in the original compound, the 

 addition-crossovers may be made rec- 

 ognizable, and by this means it is pos- 

 ible to produce such new synthetic 

 alleles as R (Stippled-Navajo), and so 

 forth. In this manner, it would be ex- 

 pected that more complex clusters 

 would develop by successive steps, un- 

 less the gene is one whose action sets 

 a closer limit on the viability of its 

 duplications. 



The great variety of genotypes that 

 might be expected to represent pos- 

 sible members of the allelic series may 

 be illustrated by a few examples as fol- 

 lows: 



Alleles (2) and (4) would be of the 

 standard /?'' type, (3) would be of the 

 dilute R' type, (1) would be of the 

 R<' type, and (5) would be a spotted 

 aleurone type with plant color. In gen- 

 eral, the differences between the alleles 

 are due to extragenic, rather than in- 

 tragenic, alterations, but this is not 

 necessarily true of the phenotypic dif- 

 ference between (4) and (5). 



With regard to the relationships be- 

 tw^een the genie elements of the com- 

 plex, the concepts of allelism and locus 

 have little meaning. All members of 

 the complex are homologous with one 

 another; presumably all have arisen 

 throujrh a long series of mutations 

 from some single ancestral gene. In a 

 sense, all may be considered allelic to 

 one another. For example, the question 

 "Is S" (the seed-color element in R^^) 



