[Heterochromatin and chromosomal polymorphism]

The structure and sizes of different short deletions of Drosophila melanogaster X-chromosome: left break in the region of locus y-ac-sc, right break in different parts of the heterochromatic region (HR) near the centromere, of the nuclei of salivary glands and metaphases of oogonia were compared. It is suggested, that the development of large blocks of deoxynucleoproteins by a small number of loci of the HR, observed in mitotic chromosomes, is due to their ability to undergo additional replication, as it previously has been shown for locus bobbed. HR in human chromosomes provide high polymorphism of the chromosome sets. The same chromosome can appear in several varieties within a population. The varieties differ in sizes of the HR and their ability for intensive fluorescence. The appearance of these heredical varieties are conditioned by deletions, duplications and inversions in the HR. Studies on revealing boundaries between normal and pathological HR polymorphism of human chromosomes are in progress.