The Analysis of a Case of Continuous Variation in Drosophila by a Study of Its Linkage Relations

Abstract

The character under consideration is that of Beaded wings in Drosophila ampelophila. All gradations of form between that of normal wings (Fig. 1) and those shown in Figs. 2 to 12 occur in the stock bottles, though certain selected strains of the stock give no normalwinged offspring. When a Beaded fly is mated to a fly of a stock not carrying genes for Beadedness in its germ plasma varying percentage of the F1 offspring is Beaded. If the male parent is Beaded the majority of the Beaded offspring are usually females; and if the female parent is Beaded, the majority of the Beaded offspring are usually males. A female Beaded fly however gives a larger percentage of Beaded daughters than does a male Beaded fly. This phenomenon is repeated from generation to generation, no matter whether a given Beaded fly has come from a male or female Beaded parent, and this shows that the phenomenon is not caused by a sex-linked gene. This phenomenon is not caused by non-disjunction of a sex-linked gene, for tests of both the Beaded and Wild stocks showed non-disjunction to be a rare phenomenon. The only explanation suggested was that the male offspring were somewhat influenced to or away from Beadedness by the nature of the cytoplasm that was brought in with the egg, while females were not readily influenced in this way. A study of the F2 generation shows that the majority of the normal F1 offspring differ from the majority of the Beaded F1 offspring genetically in that normals give fewer Beaded offspring in the F2 generation than do the Beaded flies. Beaded wings showed no linkage to any sex-linked character. Approximately one half of the flies of the F1 generation of a cross between Beaded flies and flies with characters whose genes were in the second chromosome, showed linkage in the following generation to second chromosome characters, while one half of the flies did not show such linkage. The cases where linkage did not occur gave a slightly lower percentage of Beaded offspring than did those where linkage was present. An explanation of these phenomena is sought in the suggestion that there was in the second chromosome a gene, here called 1, that was recessive but that in the heterozygous condition intensified the dominance of another gene, called B', which was not in the second chromosome. This gene 1 behaves as a lethal factor preventing the development of any fly that carries it in a homozygous condition. All of the F1 offspring of the crosses of Beaded flies by flies with characters caused by genes in the third chromosome showed linkage in the following generation between Beaded wings and the third chromosome characters. This was taken to signify that there was in the third chromosome a non-lethal gene concerned in the development of Beaded wings. This gene was called B'. This gene was shown to be the essential germinal factor in the production of Beaded wings. It is sometimes dominant and sometimes recessive. The determination as to whether B' should be dominant or recessive seems to lie in several possibilities: 1st, the nature of the egg cytoplasm; 2d, the presence or absence of the gene 1; 3d, the nature of the environmental conditions. With reference to environmental conditions, it was shown that a larger percentage of the F, generation had Beaded wings when the culture was wet than when it was dry; and more when the food was alkaline than when it was acid. No other environmental factors were discovered which influenced the production of Beaded wings. Selection of more or less extreme Beaded flies very quickly moves the average Beadedness of the offspring in the direction of the selection, but this selection apparently becomes further ineffective in a very few generations. Mutation is of very frequent occurrence in the Beaded stock and the new mutants obtained have in most cases shown themselves to be produced under the influence of one normally Mendelizing gene.