A few years ago (J. Hered. 69:27-36, 1978) the author published a formula (p. 31) for calculating the number of crossovers between a B-A translocation breakpoint and a proximal recessive gene, using F2 data from hypoploid plants. Although it was realized that movement of normal and A-B chromosomes to the same pole would give spurious evidence of a crossover event, such events were believed to be too rare to affect calculations. Occasional cases of nondisjunction of short chromosomes derived from B-A translocations recently prompted a reexamination of the normal kernels used to make the calculations of crossover frequency reported in MNL 47:145-147.
Plump kernels from wx wx du/+ hypoploids from TB-10La and TB-10Lb were planted in sand and root tip counts from all resulting seedlings made. The 10-year-old seed germinated poorly. All four seedlings from self-pollinated hypoploids of TB-10La had 20 chromosomes, as expected, but two of the four seedlings from TB-10Lb hypoploids had 21 chromosomes. Although the 10-B chromosome was not actually observed, the 21-chromosome plants were probably partial trisomics (10 10 10-B) resulting from transmission of 10 10-B gametes.
Although the data are preliminary, it is likely that small chromosomal elements occasionally nondisjoin at meiosis. The breakpoint of TB-10Lb is probably less than one-third of the distance from the centromere to the end of 10L. The 10-B chromosome thus consists of the tiny short arm of 10 and less than one-third of the long arm, plus a portion of the long arm of the B chromosome. Presumably the short 10-B sometimes fails to pair with the normal 10 and the resulting univalents either move at random to the poles or remain on the equatorial plate and are lost. Consequently, when short chromosomal elements are involved, the formula in question will overestimate the crossover rate.
J. B. Beckett
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