Excessive variation in radiation-induced chromosome aberrations

IN recent work on radiation-induced chromosome aberrations in mammalian cell cultures it was observed that the number of aberrations per cell did not follow the anticipated Poisson distribution law¹. This is clearly seen in Table 1 and Fig. 1. With Poisson variation the observed average number of aberrations at each dose-level (X̄ = ΣX/n) would have tended to equal the observed variance (S² = σ(X-X̄)²/(n-l)). Instead the variance is consistently greater, exceeding the average by a factor of 2.65 at the highest radiation dose-level used. Associated with the excessive variation noted was a surplus of cells in which there were either none or very many aberrations when comparison is made with the anticipated frequency of such instances, assuming Poisson variation. Such effects are also noted in the work of Wakonig and Ford² and Bender and Wolff³.