Influence of false twins on the ratios of twin and single sister chromatid exchanges

Abstract

The relative frequency of single and twin sister chromatid exchanges has been of considerable interest because from it inferences have been made about the structure of chromatid subunits. The results of different experiments, however, yield greatly different ratios. Although the relative frequencies of single and twin exchanges should be independent of the rate of exchange, the frequency with which two singles will occur by chance at approximately the same chromosomal position, and thus mimic a twin, will depend upon that rate. The rate of exchange has indeed varied from one experiment to another, and so the contribution of false twins might account for some or all of the observed variability. Furthermore, the frequency of such false twins may have a large effect upon the single to twin ratio since not only do false twins increase the observed number of twins, but also they reduce the observed number of singles, thus affecting both numerator and denominator of the ratio. A formula for estimating the frequency of false twins has been derived on the assumption that exchanges occur at random along the length of the chromosome. When this is used to correct the published data, it is found that most of the variability from experiment to experiment remains. Evidently, there is some other factor that influences the relative frequencies of single and twin sister chromatid exchanges. The corrected ratio of single to twin sister chromatid exchanges in endoreduplicated cells is about 4:1, a value that is closer to the ratio expected for random rejoining of subunits under certain assumptions than for “polarized” rejoining. The implication of the observed ratios for chromosome structure is thus obscure.