A Study on the Closely Linked Genes, Miniature and Dusky, in <i>Drosophila virilis</i>, with some Considerations on the Allelism of Genes

Abstract

(1) The phenotypes of various miniatures and duskies, m1, m4, m7. m10, dy1, and dy4, found in D. virilis, have been studied by measuring the wings of female flies as indicated in Fig. 3. The absolute lengths have been reduced to lengths, relative to the intercostal length. The results are shown in Table 1.(2) m1 gives by far the strongest effect, the effect of the other miniature genes decreasing in the order of m1, m7. m4, m10; the effect of dy1 is slightly stronger than that of dy4.(3) All the mutant genes are incompletely recessive to the wildtype; the heterozygotes are more or less different from both homozygotes and wild-type. The degree of this difference is roughly parallel to the degree of divergence between the homozygotes and the wild-type.(4) Of the compounds of the mutant genes, those with m1 are all intermediate between m1 and the homozygote of the other gene; m4/m7 and m7/m10 are nearly identical with m4/m4 and m10/m10 respectively, while m4/dy1 and m4/dy4 depart to a smaller degree from the wild-type than do the respective homozygotes, m4/m4, dy1/dy1 and m4/m4, dy4/dy4, from the latter. These findings indicate the allelism of the different kinds of miniatures (or duskies) and are non-allelism of miniature and dusky.(5) From m7/dy1 females, a few crossover wild-type males have been obtained, the recombination value being 0.115 percent in one series of experiments and 0.0336 in the other series.(6) Thus, miniature and dusky are mutants of different loci very closely linked and having similar characters.(7) The relation between miniature and dusky is compared with that between achaete and scute in D. melanogaster.(8) Since the recombination values mentioned above probably represent about the lowest possible normal value, they can be made the basis from which the number of gene loci separable by crossing-over in this species may be calculated. This is about 1480 for the X-chromosome and 8000 for the whole haploid chromosome set, according to one series of experiments, and about 5060 for X and more than 25000 for the whole set, according to the other. The estimate given first roughly coincide with those previously made for D. melanogaster and are probably more accurate.