SECTION 1 



COMPLEX LOCI 



1.1. Genetic Fine Structure of the rosy Cistron in 

 Drosophila melanogaster. A. Chovnick, 

 A. Schalet, and R. P. Kernaghan (Storrs, 

 U.S.A.). 



A highly efficient crossover selector system^ 1 ) 

 has been used to examine the fine structure of 

 a single cistron in Drosophila. This system per- 

 mits routine large-scale sampling approaching 

 that used in microbial studies, and permits 

 distinction between rare crossovers and spon- 

 taneous mutations. Such studies, involving a 

 large number of mutants of the rosy cistron, 

 permit the following general conclusions: (a) 

 The mechanism of recombination in higher 

 organisms, like that of microorganisms, does 

 permit crossing over within a cistron. (b) The 

 resolving power of such analysis of genetic 

 fine structure in Drosophila approaches that of 

 microbial systems. 



of phenotypes exhibited by mutations in the 

 rosy region does not include complementary 

 recessive visibles and lethals or non-complemen- 

 tary lethals: 1. Lack of complementation among 

 all mutants at separable sites and failure to 

 detect mutants complementing rosy 2 among 24 

 X-ray-induced rosy mutants selected over a 

 deficiency for the entire rosy region. 2. Absence 

 of rosy mutants which are lethal and unassociat- 

 ed with genetically detectable alterations involv- 

 ing closely neighboring loci among a sample of 

 13 rosy mutants with lethal effects localized to 

 the vicinity of the rosy region. 3. Absence of 

 complementary recessive mutations within the 

 rosy cistron among a sample of 1 1 lethals and 

 visibles selected within a small (0.5 map units) 

 chromosomal segment which included the rosy 

 region. 



1. Chovnick et al., Amer. Nat. 96, 281, 1962. 



1.2. On the Phenotypic Manifestation of Mutations 

 in the rosy Region of Drosophila melanogaster. 



A. Schalet, R. P. Kernaghan, and 

 A. Chovnick (Storrs, U.S.A.). 



According to the suggestion of Welshons 

 (1962) discontinuous mapping of complex loci 

 in Drosophila may be a function of the type of 

 mutant (recessive visibles) usually used. His 

 evidence shows that Drosophila results may be 

 reconciled with continuous mapping of mutant 

 sites in microorganisms when methods are 

 used to detect mutable sites for recessive lethals, 

 recessive visibles with atypical phenotypes and 

 possibly wild-type iso-alleles. While Welshons' 

 proposal may apply to many of the complex 

 loci in Drosophila, evidence from analysis of 

 the rosy region reveals that within a single 

 functional unit defined on the basis of a visible 

 and enzymatic phenotype, recessive visibles 

 give continuous recombination mapping of 

 mutable sites upon appropriate sampling. The 

 following considerations suggest that the range 



1.3. Notch Pseudoalleles in Drosophila melanogaster. 



W. J. Welshons, E. S. Von Halle, and 

 B. J. Scandlyn (Oak Ridge, U.S.A.). 



The Notch locus is a complex one composed 

 of pseudoallele recessive visibles superimposed 

 upon an array of pseudoallele recessive lethal 

 Notch (N) mutants. The linear order from left 

 to right on the X chromosome of all pseudoalle- 

 les is fa -fa ao - A/ 40 - N mc - spl - N 103 - N> 24 - N Co - 

 N% n -nd. The order of Ns" and nd is tentative but 

 both are to the right of yV Co . The genetic length 

 of the region has been estimated to be 0.11 units. 



It was noted previously that no Ws had been 

 discovered in the region to the left of fa no , but 

 it was presumed that they would eventually be 

 found there. Recent experiments performed 

 with N en , spl, and fa yielded results which 

 placed N el1 0.03 units to the left of fa, and in- 

 creased the genetic length of this complex locus 

 to 0.14 units. 



Similar experiments utilizing TV 39 , fa and spl 

 localized N 39 around fa no . Since TV 39 was known 

 to be cytologically deficient for salivary band 

 3C7, 85,600 chromosomes were tested from 

 N 39 lfa no heterozygotes, but no recombinants 

 were found. It is probable that deficiency N 39 

 includes the locus of fa no . 



