Figure 3.

The significance of prion heterozygosity: the unusual interactions between some Sup35 G⁵⁸X mutants. In this example the suppressible mutation is ade1–14 (UGA) and there is no tRNA suppressor in any of the strains. [PSI⁺] cells form white colonies while [psi⁻] cells form red colonies. Shown are colonies arising from single zygote cells picked from mixes of the haploid MATa and MATα strains as indicated. The four parent haploids differed at the Gly58 residue of the protein; + is G⁵⁸ (wild type) and ‘W’ is a G⁵⁸W mutant. All four parent haploids taken from a single tetrad are [PSI⁺]. Unlike normal [PSI⁺] x [PSI⁺] crosses involving wild-type Sup35 parents, the diploid progeny heterozygous for the two sup35 alleles are clearly [psi⁻]. The epigenetic segregation makes its own interpretation of the underlying Mendelian ratios.