The inheritance of the killer character in yeast

1. First- and second-generation crosses between killer, neutral and sensitive strains of yeast have been carried out in all combinations. 2. The results of this analysis indicated that the killer character is under the control of two types of cytoplasmic genetic determinant. One type, (k), determines killing, and the other, (n), neutrality. The absence, (o), of both types of determinants confers the sensitive phenotype. 3. That both types of cytoplasmic determinant require the same dominant nuclear allele, M , for their maintenance has been indicated in two ways. First, both types are lost when the nuclear genotype is changed from M to m . Secondly, cells of genotype m (k) or m (n), which have been shown to occur among the first formed cells arising from spores of Mm (k) and Mm (n) diploids respectively, are unable to maintain their cytoplasmic determinants. On the other hand, spore cultures of M (k) and M (n) genotype derived from these same diploide continue to maintain the determinants. 4. Thus genotype of killer cells is M (k), of neutrals M (n), and of sensitivities either M (o) or m (o). 5. Cells maintaining both types of cytoplasmic determinant (i.e. of genotype M (k)(n) or M −(k)(n)) have been obtained by appropriate crosses, and shown to be of killer phenotype. 6. Alternative hypotheses to account for the results of this genetic analysis have been discussed.