The Individual Gene in Relation to the Chromomere and the Chromosome

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Abstract
Seven breaks in the y-sc region of the 1st chromosome of Drosophila melanogaster were obtained by means of irradiation. The positions of the breaks with reference to each of the others were found by obtaining recombinational individuals containing the part of the chromosome to the left of one break and the part to the right of the other break and the complementary combination. Only 4 surely separate positions of breakage were found. The distance apart of these breaks is of the order of the size of individual genes. The number of genes in a given region is found to be limited and can eventually be counted. The genes studied lie in the chromatic material or nodes which represent chromomeres. A large chromomere contains a cluster of genes linearly arranged and separable from each other both by breakage and by crossing-over. The small chromomeres, such as those in the 4th chromosome, are of approximately the size which the individual genes within the large chromomeres are found to be. A. chromomere may become translocated to another position. Where phenotypic changes accompanying changes in the arrangement of genes are found to be diverse, the results show that an identical phenotypic change accompanies an identical rearrangement, whereas different phenotypic changes accompany different rearrangements. This is offered as evidence in favor of the "position effect" hypothesis. The position effect fades out over greater distances but can extend over several genes and over more than one chromomere in some cases.