Dependence of the numbers of 2-break chromosome aberrations on the size of the dose of ionizing radiation

Abstract

Rigorous mathematical expressions are derived which give the number of pairs of chromosome breaks which are separated by a distance less than h ( a) when the breaks lie on the same straight track of an ionizing particle and ( b) when the breaks are randomly dispersed throughout the cell nucleus. A third expression covers the case where both circumstances are of about the same importance. This should apply with negligible error when the mean number of tracks which cross a nucleus exceeds 5 but not when this number approaches unity. An exact method for the last case is given but it requires a computer. Application to the published numbers of 2-break chromosome aberrations produced by 3 MeV neutrons in Tradescantia microspores leads to a value of h, the distance within which breaks must be formed to produce a 2-break aberration, which is close to 1 μ. The mean separation of breaks along the ionizing tracks equals 4·8 μ and the fraction of all the cases where two breaks are produced closer than h which results in observable 2-break aberrations is 0·335. The number of primary breaks is about 10 times larger than those involved in recognizable aberrations, i.e. about 90 per cent of the primary breaks restitute.