Section 1 — Complex Loci 



1.6. A "Mutable" Gene in Drosophila melanogaster. 



William M. Hexter (Amherst, U.S.A.). 



Attached-X females properly marked and 

 heterozygous for a specific allele of garnet 

 (g 53d ) and any other garnet allele (g x ) were 

 singly mated by appropriately marked males, 

 and the female progeny were screened for the 

 presence of wild type (non-garnet-eyed) females. 

 In a total of 282,000 females, 15 were wild 

 type. In a similar test differing only in the homo- 

 zygosity of g 53d , wild type females were recover- 

 ed in 539,000 females. Of the 15 wild type fe- 

 males, 6 were non-diagnostic as to generating 

 mechanism; of the remaining 9, 6 could be inter- 

 preted as due to single crossover between pseu- 

 doallele loci. The remaining 3 are not so easily 

 explained. If the pseudoallele hypothesis were 

 correct it should be possible to demonstrate a 

 double mutant. Four of the presumptive 6 

 females were tested in a manner to reveal one 

 of the two garnet alleles should the double mu- 

 tant exist. The sum of these 4 experiments was 

 428,000 flies with single garnet mutants re- 

 covered. An additional and independent test of 

 a presumptive double mutant was to place the 

 supposed double mutant chromosome (g 53d g x ) 

 in apposition to an appropriately marked g 53d 

 free-X chromosome. Due to homozygosity of 

 g53d sucn a test would no t be expected to yield 

 any wild type progeny. In fact, however, 22 

 wild types were recovered in 685,000 flies; of 

 these 22, 11 were associated with recombina- 

 tion and could be simply interpreted as a cross- 

 over between pseudoallele loci. The remaining 

 11 were non-recombinants for outside markers 

 and are not as easily explained. The conclusion 

 based on these results is that some, if not all, 

 of the wild types were due to mutability of 

 g53d w hen heterozygous with other garnet alleles. 

 Further genetic tests have not suggested, nor 

 ruled out, the possibility that g 53d is a duplica- 

 tion. 



segment of the forked locus) might be due to 

 duplication of the right forked sub-locus at a 

 pre-meiotic division, a suggestion which would 

 account not only for the relatively high rever- 

 sion rate of f 3N (to f 3N+ ), but also for the 

 restriction of the reversions in large measure 

 to diploid cells (oogonia and spermatogonia) 

 in irradiated material. Since a ring X would 

 get lost much more often than a rod as the 

 result of a duplication in a dividing chromosome, 

 the recovered reversion rate of f 3N would ex- 

 pectedly be higher for a rod than a ring X, pro- 

 vided the reversions were due to duplications 

 (whether X-ray induced or spontaneous). In 

 accordance with the above suggestion, it has 

 been found that the spontaneous reversion rate 

 of f 3N is much higher in a rod X than in a ring as 

 follows. Among 380,000 females heterozygous 

 for a rod and a ring (but homozygous for f 3N ) 

 there were 12 reversions recovered in the rod and 

 one in the ring. Insofar as these results would 

 indicate that the forked locus arose as the result 

 of a duplication, they would support the Lewis 

 theory of pseudoallelism in Drosophila. 



Detailed data are in press in Proc. Nat. Acad. 

 Sci. Wash. 



Supported by a grant from The National 

 Institutes of Health (CA-03 1 14). 



This investigation was supported by U.S. 

 Public Health Service Research Grant GM 

 08889-02 from the Division of General Medical 

 Science. 



1 .8. The Genetic Fine Structure of the Mutants 

 /."' and /.' in Drosophila melanogaster. 

 B. H. Judd (Austin, U.S.A.). 



The zeste locus in Drosophila melanogaster is 

 located at 1.0 on the X chromosome. The mu- 

 tant z m (zeste-mottled) was found by Green as a 

 single male (sc z m ) from the cross sc z ec ctj 

 w M x sc z ec ct. Later z 1 (zeste-light) was found 

 by Becker as a single male in the sc z m stock. 



Analysis of z m leads to the hypothesis that it 

 does not represent a change at the zeste locus, 

 but is the result of an asymmetrical exchange at 

 the white locus. The white locus is very closely 

 linked to zeste (1.5 on the X chromosome) 

 and is functionally related. It is postulated that 

 z 1 arose as a similar asymmetrical exchange from 



1.7. Evidence from Rod-ring Experiments for the 

 Duplicational Origin of f 3N Reversions in Dro- 

 sophila. Edgar Altenburg and Luolin S. 

 Browning (Houston, U.S.A.). 



It was previously suggested that the rever- 

 sion of f 3N (an allele located in the left sub- 



The change in the white locus can be localized 

 within the two rightmost recombination sites 

 of the locus. The phenotypic expression of this 

 change leads to the conclusion that a portion 

 of the white locus has been duplicated. Recom- 

 bination experiments give support to this inter- 

 pretation. 



The duplicated nature of the two mutants 



