Chromosome aberrations were scored in swine leukocytes after an in vitro exposure to 300 R of gamma radiation. Six pigs were sampled periodically from 4 through 72 weeks of age; a second group was sampled at 11.0 and 11.5 years. Coefficients of two-hit aberration production were relatively high in young animals, fell somewhat as the animals matured, and increased slightly with advanced age. Although the coefficients for one-hit aberrations remained fairly constant, the contribution of achromatic lesions within this category was significant among young pigs but nonexistent in mature animals. Among the control samples, spontaneous chromosome deletions rose slightly in cells from mature animals; other types of anomalies showed no relationship to animal age. Chromatid aberrations, which are not normally considered to be radiation-related in these cells, showed increases among irradiated samples from older pigs.

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