| Summary: | The effect of altered histone gene multiplicity on chromatin structure was assayed by measuring changes in gene activity associated with position-effect variegation. Heterozygous deficiencies of the histone gene cluster of Drosophila melanogaster increased the proportion of cells within a tissue in which a variegating gene was active. This effect was not dependant on the Y chromosome and applied to both X-linked and autosomal variegating genes. Deletions of the histone gene complex imposed on different source chromosomes elicited the same response. Partial deficiencies, which delete different regions of the cluster, varied in their effect on variegation. Duplications of the histone gene cluster did not increase the proportion of cells in which a variegating gene was inactive. The presence of deficiencies or a duplication of the cluster in the maternal genome did not modify the extent of position-effect variegation in their progeny. These results are discussed with respect to current knowledge of the organization of the histone gene cluster and control of its expression. === Medicine, Faculty of === Medical Genetics, Department of === Graduate
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