The predicted ratios of single to twin sister chromatid exhanges

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There are two kinds of sister chromatid exchanges: those that effect only one of the chromosomes derived from a common parent (single exchanges), and those that effect both (twin exchanges). By making assumptions about the structure of a chromatid and the way in which sister chromatid exchanges are formed, one can predict theoretically various relative frequencies of the two kinds of exchange. Consequently, from the ratio of the observed frequencies of single and twin exchanges, one can make inferences about chromatid structure. In the original experiments performed by Taylor (1958) tha data, although very variable, were closer to the expectation is the subunits behaved differently from one another than the expectation if they were identical. This indicated that chromatid behavior paralleled the behavior expected from a DNA double helix. This parallel has often been considered evidence that a chromatid is a single-stranded structure. In this paper it is shown that other predictions of the ratio of single to twin exchanges can be made if it is assumed that the exchanges are induced by a one- or a two-hit process by the radiation from the [ 3H]thymidine used to detect them. One of these ratios is very close to much of the more recently published data, although not to the original data. Since this ratio is derived from the assumption that the chromatid subunits are similar to one another, it is concluded that the relative frequencies of sister chromatid exchanges do not at present provide any basis for distinguishing between single and multistranded chromatids.