Length and distribution of meiotic gene conversion tracts and crossovers in Saccharomyces cerevisiae.

We have measured gene conversion tract length in strains of the yeast Saccharomyces cerevisiae containing three to six restriction site heterozygosities in a 9-kb interval. Tetrads containing a conversion were identified genetically by nonmendelian segregation of a marker in the middle of the interval. Gene conversions accompanied by a crossover have a tract length of 1.4 kb +/- 0.7 kb, which is indistinguishable from a tract length of 1.6 +/- 0.8 for conversions without an associated exchange. Among tetrads identified first as having a crossover in the interval, the average gene conversion tracts were apparently significantly shorter (0.71 +/- 1). We provide evidence that this apparent difference is due to the method of measuring conversion tracts and does not reflect a real difference in tract length. We also provide evidence that the number and position of restriction site markers alters the apparent distribution of the conversion tracts. More than ninety percent of the conversion tracts spanning three or more sites were continuous.