5. Chromosome segregation in heterozygous translocations.


Segregation from chains appears to differ from that in rings. In the chain‑forming T1‑6b and T5‑6b heterozygotes (break in satellite) with short interstitial segments (region between centromere and translocation break), very little adjacent 2 segregation (homologous centromeres go to the same pole) occurs. In rings with short interstitial segments adjacent 2 segregation occurs in 14 to 36% of the sporocytes (differing in different translocations). In chains with long interstitial segments, adjacent 2 segregation also appears to be very low as shown by T5‑6c/+ which forms chains at diakinesis in 30 to 40% of the sporocytes, yet has little, if any, of this type of segregation.


If chromosomes that crossover go to opposite poles, as the evidence indicates, then a species with directed segregation would show varying degrees of sterility associated with a ring of 4, the amount dependent on the frequency of crossing over in the interstitial segments, the maximum being 50%; since half of the spores in each quartet, which follows such crossing over, abort.


In the search for directed segregation, translocations with short interstitial segments are being used in crosses with material of different origin.


C. R. Burnham