X-ray-induced lethal sectoring, a frequently occurring type of dominant lethal damage, was studied in diploid yeast by pedigree analysis, in which the fate of various vegetative lines of descent from individual irradiated cells was determined. A propensity for sectoring was associated with a reduced rate of cell division, and lines of descent in which lethal sectoring persisted indefinitely could be isolated. The lethal sectors produced by an unstable line usually consisted of only one or two dead cells. The degree of instability, as measured by frequency of sectoring, was variable, and it was possible to isolate sublines with greater or lesser degrees of instability by a technique of selective serial transfer. Mechanisms which may be responsible for lethal sectoring are discussed, and it is concluded that one of the more likely of these involves the misassortment of extrachromosomal entities at cell division.

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