Expression of cryptopleurine resistance in Saccharomyces cerevisiae

An examination of gene expression in diploids may not always be sufficient for determination of the dominant or recessive character of an allele. In Saccharomyces cerevisiae resistance to cryptopleurine has been attributed to a single recessive nuclear gene, cryl, located on chromosome III. We found, contrary to expectations, that resistance to cryptopleurine is not expressed in diploids that are monosomic for chromosome III. Examination of strains of different ploidy on gradient plates shows that the presence of the sensitive allele in a cell does not affect the level of resistance, but rather the level of resistance is directly related to the ratio of resistant alleles to the number of chromosome sets.