Preferential reduction of dicentrics in reciprocal exchanges due to the combination of the size of broken chromosome segments by radiation.

Induction rates of the dicentrics and translocations involving chromosomes 2 and 4 in peripheral lymphocytes irradiated with X-rays at a dose of 3 Gy were examined using a conventional Giemsa staining method and a chromosome painting method. In total, 228 reciprocal exchanges detected in 982 metaphases were classified into three groups according to the break points of the original chromosomes. The incidence of both acentric fragments being larger than half of the original chromosome (combination 1) was only seven (3%) and did not contribute significantly to induction rates. When the broken acentric fragments of two affected chromosomes were smaller than half of the original chromosomes (combination 2), which was found in 175 (77%) rearrangements, the induction rates of dicentrics and translocations were about the same (86:89). But if the sizes of the broken segments were unequal in both chromosomes (combination 3: one with a larger acentric part and the other with a smaller acentric part), the yield of dicentrics was significantly lower than that of translocations (16:30). It was suggested that there was a special mechanism causing preferential reduction of dicentrics in reciprocal exchanges originated from the heteromorphic size of broken chromosomes in the last combination.