Chromatid aberrations induced by gamma irradiation. II. Non randomness in the distribution of chromatid aberrations in relation to chromosome length in Vicia faba root-tip cells.

I N the experiments on Vicla faba roo:-tip cells reported in the first paper in this series (EVANS 1961), it was observed that the frequency with which the chromatids of various chromosomes, or chromosomc arms, underwent interchsnge followiny: irradiation was not in direct proportion to the metaphase length of these chromosomes. Taken as a group, the ten acrocentric chromosomes, compared with the two metacentric chromosomes, were found to have taken part in interchange more often than would have been expected on the basis of their relative lengths at metaphase, and it was thought that this deviation from expectation between chromosomes or chromosome groups might in fact be a consequence of a non random distribution of interchanges within the chromosomes. The localization of aberrations induced by certain chemical treatments to particular regions of chromosomes is well known, and this is especially so in the case of chemical treatments of Vicia faba roots (FORD 1949; REVELL 1953; and MCLEISH 1953). A non random distribution of aberrations within chromosomes has also been reported for radiation-induced aberrations in other organisms (for some of the early papers see review by SPARROW 195 1 ) , and since the completion of the present work non randomness in the distribution of radiation-induced breaks has been reported between chromosomes of a variety of Vicia faba (REIGER and MICHAELIS 1959). In the present paper a study has been made of the frequency-distributioii of radiation-induced isochromatid and interchange aberrations both within and between chromosomes of diploid cells. Additional information is also presented on the distribution and the frequency with which interchange occurs between homologous chromosomes in tetraploid cells, these cells being irradiated in the latter part of the interphase following their induction by colchicine treatment (EVANS 1961 ).