During vegetative, asexual reproduction in heterozygous Tetrahymena thermophila, the macronucleus divides amitotically to produce clonal lineages that express either one or the other allele but not both. Because such phenotypic assortment has been described for every locus studied, its mechanism has important implications concerning the development and structure of the macronucleus. The primary tools to study assortment are R(f), the rate at which subclones come to express a single allele stably, and the output ratio, the ratio of assortee classes. Because R(f) is related to the number of assorting units, a constant R(f) for all loci suggests that all genes are maintained at the same copy number. Output ratios reflect the input ratio of assorting units, with a 1:1 output ratio implying equal numbers of alleles at the end of macronuclear development. Because different outcomes would suggest a different macronuclear structure, it is crucial that these parameters be accurately measured. Although published R(f) values are similar for all loci measured, there has been no commonly accepted form of presentation and analysis. Here we examine the experimental determination of R(f). First, we use computer simulation to describe how the variability inherent in the assortment process affects experimental determination of R(f). Second, we describe a simple method of plotting assortment data that permits the uniform calculation of R(f), and we describe how to measure R(f) accurately in instances when it is possible to score only the recessive allele. Using this method to produce truly comparable R(f)s for all published data, we find that most, if not all, loci assort at R(f)s consistent with approximately 45 assorting units, as has been asserted.
