Chromosome disjunction   (See News Letter 19:31. 1945).

 

In plants heterozygous for T 5‑6c, the low percentage of crossing over with the chromosome 5 inversion in the translocated chromosome as compared with the amount observed with the inversion in the normal chromosome can now be explained without resorting to "position effect". When Dr. A. H. Sturtevant saw the data, he suggested that the cytological data on crossing over (percentage frequency of the‑ crossover type or "half disjunction” quartet) did not measure crossing over within the inversion in both cases. When we drew the chromosomal diagrams (checked later) they showed that this was trul. When the inversion is in the translocated chromosome, crossovers within the inversion do not give rise to the cytologically recognizable “half-disjunction" quartets; whereas when the inversion is in the normal chromosome these crossovers are recognizable in that manner. In the one case these quartets result only from crossing over between the translocation break (center of the cross) and the new position of the centromere, consequently comparable to that in the stock heterozygous T 5‑6c but homozygous for the inversion.

 

C. R. Burnham