Localization of the Genetic Factors Responsible for the Kinetic Activity of X Chromosomes of Drosophila Melanogaster.

HE genetic analysis of kinetic activity in Drosophila melanogaster based T on the fate of first anaphase bridges derived from double crossing over within heterozygous inversions made it possible for centromeres to be characterized as weak or strong. The experimental basis for the distinction between these two types was described earlier (NOVITSKI 1952) and a more detailed description of the genetic consequences of anaphase bridge formation at both first and second meiotic divisions is presented elsewhere (NOVITSKI 1955). Centromeres derived from a normal X chromosome are weak; whereas those found on detachments of attached-X chromosomes are strong when they carry the long arm of the Y chromosome. Some centromeres carrying the short arm of the Y chromosome are weak and some are strong. In general, the method of replacing one centromere with another involves heterochromatic exchange, with a consequent alteration of the associated heterochromatin. Of considerable genetic interest would be a determination of whether the genetic results that have been attributed to “centromere activity” depend on the nature of the centromere itself, or on the constitution of the adjacent heterochromatic regions from which the centromere is ordinarily inseparable. The experiments to be described here, designed to distinguish between these two possibilities, indicate that the latter is true.