Section 1 — Complex Loci 



Obviously the region left of fa no is capable of 

 producing recessive lethals as indicated by the 

 localization of N eU - Furthermore, the ./V 39 ex- 

 periments indicate that the single salivary band 

 C37 cannot contain all the pseudoallele loci 

 comprising the functional Notch locus since 

 mutants like A/ e " and N Co are localized at appre- 

 ciable distances to the left and right of the de- 

 ficiency. 



as found by Green at the white locus, in which 

 event the resulting duplication in the present 

 case might be spectacle and the complementary 

 deficiency a lethal. 



This investigation was supported by U.S. 

 Public Health Service Research Grant 

 GM-8889-02 from the Division of General 

 Medical Science. 



1.4. An Apparent Non-equivalence in Crossover 

 Rates between Lozenge Alleles in Trans and 

 Cis Arrangement in Drosophila. Luolin S. 

 Browning and Edgar Altenburg (Houston, 

 U.S.A.). 



The rate of recovered crossovers between 

 lz BS and lz 46 in Drosophila was found to be 

 significantly greater from females heterozygous 

 for these two lozenge alleles in trans arrange- 

 ment as compared with cis as follows. Among 

 350,000 Fi from the trans arrangement (lz BS + / 

 + lz 46 ), 116 crossovers (46 lz BS lz 46 and 70 + +) 

 were recovered, or about 1 crossover in 3000 

 offspring; among 436,500 Fi from the cis ar- 

 rangement (lz BS lz 46 / + +) 75 crossovers 

 were recovered (75 being the combined number 

 of lz BS + and + lz 46 , both of lozenge phenotype 

 as contrasted to the spectacle phenotype of the 

 double mutant non-crossover class on the one 

 hand and the complementary normal on the 

 other), or about 1 crossover in 5820 offspring. 

 It is not considered very likely that this difference 

 in rates could be due entirely to the relative 

 viabilities of the crossover classes as compared 

 with the non-crossovers in the trans and cis 

 arrangements. Work earlier reported indicated 

 that the crossover rate between apricot (w a ) 

 and white (w) was also greater for the trans than 

 the cis arrangement. In explanation of these 

 results, it is suggested that alleles might some- 

 how differ structurally. One series of lz-locus 

 experiments (involving a question as to the 

 identity of the alleles) gave an unexpected re- 

 sult in that the two lz alleles in trans arrange- 

 ment gave the apparent double mutant class 

 (10 phenotypic spectacles in 70,000 offspring) 

 but none of the complementary normal pheno- 

 type. Judd reported a result which we interpret 

 as similar to this, insofar as in the Fi from apri- 

 cot and buff in trans arrangement, the only 

 "exceptions" found were of white phenotype 

 (no normals). We have no explanation for this 

 kind of result except perhaps that it is due to 

 some type of non-homologous pairing such 



1.5. A Study of Recessive Lethals on the Dot-like 

 Fourth Chromosome in Drosophila melano- 

 gaster. Benjamin Hochman (Knoxville, 

 U.S.A.). 



A three-year study of 1352 fourth chromo- 

 somes, extracted from natural populations of 

 D. melanogaster, has uncovered 15 which are 

 lethal in the homozygous state. Allelism tests of 

 14 of the lethals (one was lost) demonstrate that 

 they occupy eleven different loci. One locus is 

 represented three times; a second locus twice; 

 and nine have a single representative each. 

 One of these nine is allelic to a lethal which has 

 arisen spontaneously in two different laboratory 

 stocks. 



Crosses of the lethals to the six existing 

 chromosome 4 dominant visibles have thus far 

 localized one of them to the bt u locus and a 

 second to that section of the microchromosome 

 delimited by the Minute-4 deficiency. None of the 

 lethals permits pseudodominance of any of the 

 seven non-allelic recessive visibles tested. 



Several X-ray-induced lethal fourth chromo- 

 somes, generously provided by Drs. M. M. Green 

 and H. Gloor, have been examined for inter- 

 actions with the lethals of spontaneous origin. 

 Three heterozygous combinations which are 

 lethal have been observed to date. 



If all of the spontaneous microchromosomal 

 lethals are "point" mutations, the number of 

 loci identified on chromosome 4 has been ap- 

 proximately doubled over the figure based here- 

 tofore solely on the known visible mutations. 

 The total number of "potentially-lethal" loci 

 to be expected on the microchromosome, as 

 well as the developmental period during which 

 each lethal acts, will be discussed. 



This research was supported by Grant 

 RG 9845, United States Public Health Ser- 

 vice, National Institutes of Health. 



