Figure 1. The four basic aberration categories. Examples of common two-lesion forms are shown as they might appear at metaphase when solid staining is used: A, asymmetrical rejoining; S, symmetrical rejoining. Note that some S aberrations are not visible when solid staining is used. SU, sister union with both ends laterally fused; NUp, nonunion proximal; NUd, nonunion distal, lateral fusion absent in the centromeric or acentric portion.

Figure 2. A group of five chromosomes from a cell irradiated in G₀. (a) Solid stained. A chromosome-type dicentric plus an acentric fragment is visible. The obvious inference is that these two elements are related and are the result of a two-break asymmetrical exchange (cf. Figure 1). There might also be symmetrical changes present in other chromosomes, but these are not registered with solid staining. (b) Two different chromosomes have been FISH-painted distinctively, and the remaining chromosomes counterstained with a nonspecific fluorochrome. The dicentric pattern is clearly not simple, for the terminal segment is not in the acentric fragment. At least three breaks in three chromosomes are required to obtain such a pattern. An apparent reciprocal translocation is present between two of the other chromosomes. Again the inference is that it is a simple two-break symmetrical exchange (cf. Figure 1). There is no suggestion that the two aberrations are in any way related. (c) All five chromosomes have been distinctively painted using mFISH. We can now see the complete exchange ‘configuration’ from which earlier patterns were derived. The translocation of pattern (b) is not simple. In fact, all the break-ends of the five chromosomes are involved in one grand ‘musical-chairs’ exchange: a ‘cyclical exchange of order 5’ (Sachs et al., 1999) or ‘c5’ (Cornforth, 2001).