↵^a A comparison of MET13 prototrophic random spores derived from a met13-1/met13-5 diploid (first two columns) with equivalent segregants obtained by treating the 1451 tetrads in set II as individual spores (last two columns). Only NAT-containing spores were counted so that crossover and noncrossover events were counted only once, to permit statistical comparisons with the random Met⁺ spore data.

The χ² P-values at the heads of columns 2 and 4 compare the crossovers in those columns to the noncrossovers in columns 1 and 3, respectively. The χ² P-values in the last rows of parts A and B compare the data in those columns with the total number of NAT-hph and NAT-HPH segregants in set II, treated as single spores. Random spores were obtained from a yeast diploid strain obtained by a cross between haploid strains isogenic to those used for tetrad analysis (Table 1, set II). The diploid used to generate Met⁺ random spores was derived from the haploids MATα lys5 NAT met13-1 cyh2 KAN and MATa ade5,7 leu1 HPH trp5 met13-5 crl3. The randomness of the spores was verified by monitoring phenotypes at both the ADE5,7 and the LEU1 loci (480 Ade⁻ and 465 Ade⁺; 434 Leu⁻ and 511 Leu⁺). Among Met⁺ spores, there were 466 crossovers (371 Nat⁻ Hph⁻ and 95 Nat⁺ Hph⁺) and 479 noncrossovers (370 Nat⁺ Hph⁻ and 109 Nat⁻ Hph⁺) for the flanking markers. The inequalities in both the crossover and noncrossover classes suggest that 78% of Met⁺ spores are the result of conversion of met13-1. This bias is expected from the sign and magnitude of the slope of the conversion gradient at the MET13 locus, where met13-5 is at the low-conversion end (J. McCusker and J. E. Haber, unpublished data).