Comment on the ratio of chromosome-type dicentric interchanges to centric rings for track-clustered compared with random breaks.

Evidence has been published suggesting that the ratio of chromosome-type dicentric interchanges to centric rings (D/R or F) is significantly lower for neutrons than for X or gamma rays, and it is proposed that a low D/R could be used as a "fingerprint" for high-LET radiations. One explanation offered for this observation is that the closely spaced, clustered breaks confined to linear tracks will favor intrachanges, as opposed to interchanges, leading to a lower D/R, while the more scattered, random breaks of low-LET radiations will favor the reverse situation and elevate the D/R. We have tested this suggestion empirically by constructing various modeled tracks and grids of breaks which satisfy the proposed conditions. These have then been superimposed, in random orientation, on an array of hexagons, representing a planar section through the interphase arm domains of 14 polarized, metacentric, G1-phase chromosomes, and the D/R computed from the interaction of break clusters with the arms. The ratios recovered were essentially the same for the four different break distributions tested, and we conclude that, for this simple model, the determinant of the D/R is the arm arrangement in the array, rather than the disposition of the breaks.