Section 1 — Complex Loci 



leads to further asymmetrical pairing and ex- 

 change within the locus when these chromo- 

 somes are used for crossover studies. A variety 

 of unusual recombination products presumably 

 resulting from asymmetrical exchanges have 

 been analyzed. 



1.9. Biochemical Division of the White Locus in 

 Drosophila melanogaster. Hugh S. Forrest 

 (Austin, U.S.A.). 



Despite the fact that the white locus in Droso- 

 phila melanogaster has been known for some 

 time to be divided into subloci separable by 

 crossing over and other genetic tests, there is 

 remarkably little knowledge of its fundamental 

 biochemical role. It is believed by some workers, 

 for example, that mutation at this locus results 

 in an "incomplete" or "non-functional" pig- 

 ment-synthesizing granule, resulting in loss of 

 synthetic ability for both the red and brown 

 pigments. 



In a new attempt to investigate the biochemical 

 function of the locus, it has been shown that 

 all of the alleles at the locus so far tested can 

 be divided into two groups — those that accu- 

 mulate the compound xanthurenic acid, albeit 

 in smaller amounts than wild type, and those 

 that do not contain this compound at all. It is 

 reasonable to assume that xanthurenic acid 

 can be used as an index of the production of 

 3-hydroxykynurenine, an intermediate in the 

 biosynthesis of the brown pigments. Thus the 

 white locus may be separable into two functional 

 areas, one of which, in the mutant condition, is 

 unable to produce 3-hydroxykynurenine (meas- 

 ured as xanthurenic acid) and the other of which 

 can produce limited amounts of this material. 

 A theory relating this finding with the failure 

 or partial failure of mutants of the white locus to 

 synthesize the red eye pigments will be presented. 



1.10. Complementation Groups at a Complex Locus 

 in the House Mouse. Dorothea Bennett and 

 L. C. Dunn (New York, U.S.A.). 



The study of three newly arisen alleles has 

 provided the opportunity to compare the 

 effects of lethals of independent origin, and 

 especially to compare members of the same 

 complementation group with respect to second- 

 ary effects. Complementation groups are defined 

 as consisting of alleles from different sources 

 which are lethal in combination. Alleles in the 

 same complementation group are in general 



indistinguishable; they produce similar syn- 

 dromes of abnormalities at similar times in 

 embryogeny, and have similar effects on male 

 transmission ratio and recombination. Combina- 

 tions of members of different complementation 

 groups are of course viable to some extent, but 

 the proportion of viable compounds found 

 varies greatly depending on the alleles concerned. 

 The three new alleles reported here were found 

 to belong to three different previously established 

 complementation group. Their characteristics 

 regarding embryonic effects, transmission ratio, 

 and recombination followed the general pat- 

 tern of being in each case indistinguishable from 

 other members of their group. However, one of 

 the new alleles was found to be clearly distin- 

 guishable from other members of its group on 

 the basis of degree of complementation with 

 another lethal allele. This suggests that alleles 

 in the same complementation group may not 

 have identical structures, and that quantitative 

 measurements of the degree of complementa- 

 tion amongst different alleles may be used as a 

 basis for assessing their similarity or dissimi- 

 larity. 



1.11. Genetical Analysis of the R Chromonemal 

 Region of Mormoniella. P. W. Whiting and 

 Doris J. Bush (Philadelphia, U.S.A.). 



Two eye-color factors, O and S, mutating 

 with relatively high frequency to oyster-white 

 and to scarlet respectively, make the "/?" 

 region of Mormoniella especially favorable for 

 genetical analysis. These colors serve as markers 

 for many other 7?-locus changes including semi- 

 lethals and lethals, steriles and near-steriles. 

 Complementation tests identify factor homolo- 

 gies. For factors such as lethals and male-steriles 

 which cannot be transmitted through haploid 

 males, diploid males heterozygous for these 

 deleterious factors are used. Two different 

 female-steriles have been found, masked in 

 lethal-bearing genes. Two different lethals, 

 complementary to each other and, therefore, 

 non-homologous, have proved non-complemen- 

 tary, "homologous", to a third lethal. It is sug- 

 gested that the last impairs some process essen- 

 tial to the normal functioning which is impaired 

 by each of the other two lethals. Formula of 

 each /Mocus gene is given in terms of its factor 

 states. By use of the gene formulae it is possible 

 to predict the phenotypes and breeding behavior 

 of the different compounds. It is postulated that 

 the complexity of the R region is not greater 

 than that of other chromonemal regions. Its 

 complexity has been explored and revealed by 



