Preferential Segregation in Triplo-IV Females of Drosophila Melanogaster.

H E occurrence of non-random segregation in triplo-IV females of T Drosophila melanogaster was reported in a preliminary note (STURTEVANT 1934) of which this is the full account. As there shown, and elaborated below, all the tested fourth chromosomes may be arranged in a series, of such a nature that each chromosome will (in a triplo-IV female) pass to the haplo-IV pole a t meiosis more often than will any chromosome that lies above it in the seriation. Expressed in another way, if two chromosomes (A and B) are tested against any third chromosome (C), then C will “prefer” that one of the other two that lies higher in the series. These relations are first described in the case where one of the IV chromosomes concerned carries genes from the X chromosome. The data on other triplo-IV females are then presented. An algebraic analysis that leads to quantitative predictions is then developed. It should be remembered throughout that the relations discussed are between whole chromosomes, not individual loci-a circumstance made experimentally feasible by the fact that crossing over in this chromosome is negligible in frequency. The chromosomes concerned are designated by the names of the mutant genes they carry, but these genes are to be regarded only as convenient markers, making it possible to follow the behavior of the chromosomes concerned.