The Hypothesis of Chromosomal Interference

Abstract

IT has been known for a long time that both chromatid and chromosomal interference produce similar effects on recombinations recovered in classical genetical (Mendelian) analysis in which single chromatids are sampled. Although deviations from a unitary coincidence are probably due either to chromatid or chromosomal interference, the classical method does not permit one to determine whether one or the other, or both, processes are effective in producing the deviations. It is strange, therefore, that chromosomal interference has been the accepted explanation of non-unitary coincidence since the very beginning of modern genetical analysis. The utility of either hypothesis in the analysis of single-chromatid experiments may be demonstrated by considering the following map: The expected frequency of double-recombinations would be 16.7 × 14.5 × 10⁻⁴, or 2.4 per cent, whereas the actual frequency is only 0.9 per cent. The accepted conclusion has been that one cross-over actually interferes with the occurrence of another, the ‘strength’ of this interference being indicated by the coincidence value (in this case, 2.67). The hypothesis of chromatid interference is, however, equally applicable if one supposes that adjacent cross-overs form four-strand double exchanges at a frequency equal to or greater than 72 per cent.