AN ANALYSIS OF CROSSING OVER WITHIN A HETEROZYGOUS INVERSION IN DROSOPHILA MELANOGASTER

Abstract

HE study of crossing over between chromosomes differing by an inversion provides information on the capability of chromosomes to synapse and undergo exchange despite the niechanical ohstacle imposed by the inversion loop configuration. Extensive information is available in Drosophila iaelanogaster on the frequencies of crossing over outside the inverted region of inversion heterozygotes, but measurement of crossing over within the inversion presents difficulties, since single crossovers are ordinarily lost by the formation of dicentrics or duplication-deficiency chromosomes. Frequencies of double crossing over in very long inversion heterozygotes are known ; this type of heterozygote, however, constitutes the least interesting and infortnative case because the chromosomes may pair in reverse direction without the formation of a typical inversion loop. The ideal conditions for this sort of study, then, include the use of a medium-sized inversion centrally located in a chromosome arni with an appreciable pairing segment on both sides of the inversion and some specially devised constitution such that crossovers both within and outside of the inversion yield genetically detectable products. These conditions have been fulfilled by the use of a tandem metacentric compound X chromosome heterozygous for the dl-49 inversion. An analysis of crossing over in this conipound is presented below. CONSTRUCTION O F T H E TANDEM METACENTRIC COMPOUND The compound X chromosome used in these experiments was made up according to a method described previously (NOVITSKI 1954) and the appropriate scheme is given in figure 1 of that work. Bar males were mated to females heterozygous for a chromosome in normal sequence, except that it carried the dl-49 inversion, and a second X chromosome which was in inverted order, with the Inse of I n (1) E N and the tip of I n (l) s c 4 , and which had, in addition, a short arm consisting of the proximal segment of the X chroniosoine derived from the R" translocation. Crossing over between the base of the first chromosome and the short arni of the second should give rise to a metacentric compound S chromosome with its components tandemly arranged , and an egg with such a chromosome, if fertilized by a Y-bearing sperm, would produce a non-Bar female, an exceptional type.