Radiation-Induced Genetic Segregations in Vegetative Cells of Diploid Yeast.

LTRAVIOLET irradiation frequently induces homozygosis a t one or more loci U in heterozygous vegetative cells of Saccharomyces cerevisiae. For a particular allelic pair, the observed frequency of such inductions can approach two percent among survivors under conditions where fewer than fifty percent of the irradiated cells are killed. These alterations are almost certainly due to genetic segregation rather than to gene mutation, since: (1) complementary types (homozygous recessive and homozygous dominant) often appear as different lines of descent from a single irradiated heterozygote, (2) irradiated haploids show only rare changes from dominant to recessive, and (3) irradiated homozygous recessive cells show only rare changes to heterozygous (JAMES 1953; 1955). However, the observed homozygosis could result either from independent segregations of individual loci or from linked segregations involving whole chromosomes or parts of chromosomes. The present investigation was undertaken to obtain information on the mechanism by which the segregants are produced.