6. Chromosome disjunction. In an abstract (Records Genetics Society-1944, p. 14) it was reported that chromosome disjunction in a plant heterozygous for interchange T5-6c was markedly changed when the position of the chromosome 5 centromere was shifted nearer the center of the cross by the presence of a homozygous inversion in chromosome 5. It was also reported that the amount of cytologically observed crossing-over when the inversion was heterozygous was different depending on whether the inversion was present in the interchanged chromosome 5 or in the non-interchanged 5. Cytologically the pairing configurations in the two cases should be similar. It was thought possible that some additional change might have accompanied the crossing-over by which the inversion was introduced into the interchanged chromosome 5. Accordingly a prophase study of the following homozygous stocks has been made: inversion in chromosome 5 T5-6c, and T5-6c plus inversion. Fortunately one of the breaks in the inversion and in T5-6c was in a heavy chromomere region, while the second was in a region with small chromosomes. Positions of breakage and rearrangement could be clearly recognized. The stock combining both also appeared to have the exact morphology expected. The differences in crossing over mentioned above appear to result from some other cause.

Chas. R. Burnham assisted by Gertrud Stanton