A COJIPARATIVE STUDY OF CROSSING OVER IN ATTACHED-X CHROJIOSO3IES OF DKOSOPHILA AIELANOGASTERl

HE work of L. V. MORGAN (1925) demonstrated that crossing over in attachedT X chromosomes of Drosophila melanogaster could be systematically studied if one could obtain attached-X chromosomes which were properly marked. From her experiments with these attached chromosomes, MORGAN concluded that a t least sometimes crossing over occurred when four chromatids were present. In this same paper MORGAN indicated that by observing the various kinds of attached-X offspring of an attached-X mother, one could determine the genotype of the heterozygous mother. The first experiment designed for the systematic study of crossing over in attached-X chromosomes was performed by ANDERSON (1925). This piece of work succeeded in demonstrating the following pertinent points: (1) Crossing over in attached-X chromosomes appeared to proceed normally and regularly in spite of the fact that the two chromosomes were obliged to behave as a unit. (2) Single exchanges between non-sister chromatids occurred a t random. (3) Crossing over regularly occurred a t the four strand stage and only two of the four chromatids crossed over a t any one level. (4) The homozygosis frequency of the recessive allele carried by the X-chromosomes decreased in a regular fashion as the position of the alleles approached one end of the chromosome, at which end, it was inferred, the centromere must be located. In addition to the above mentioned points, ANDERSON demonstrated in detail the method of progeny testing whereby the genotype of an attached-X female could be determined. EMERSON and BEADLE (1933) published the results of an experiment the purpose of which “was to obtain more accurate data concerning the relative frequencies of different crossover types.” These data supported ANDERSON’S conclusions and in addition made possible a comparison of the frequency of 2-strand and 4-strand double crossover types. In order to determine the ratio of these types, the investigators incorporated the data of ANDERSON and a large block of data from STURTEVANT (1931). The combined data demonstrated a 1: 1 ratio between 2-strand and 4-strand double exchanges for those cases in which the first detectable exchange nearest the spindle fiber is non-reciprocal; i.e. the exchange occurs between chromatids which are destined to be attached to different centromeres a t the second meiotic division. This 1: 1 ratio indicated that the first exchange in no way determined what type of second exchange would occur. In other words, there was an absence of chromatid interference. BEADLE and EMERSON (1935) then performed another experiment using attached-X