Topology of constitutional reciprocal translocations in metaphase.

We studied in 39 carriers of 26 reciprocal translocations (including five de novo and seven of indeterminate occurrence) the metaphase localization of the derivative chromosomes, their normal non-homologous counterparts (here called A and B), and two control pairs (C and D). In eight familial translocations, we analysed two to five carriers. We digitally captured 10 G-banded lymphocyte metaphases per individual and measured in microns the largest diameter (d) of the metaphase and six intercentromeric distances: (1) der A<-->der B (problem distance 1, pd1), (2) der A<-->B (pd2), (3) der B<-->A (pd3), (4) A<-->B (control distance 1, cd1), (5) the smaller distance between C and D (cd2) and (6) the largest distance between C and D (cd3); in addition, the average between C and D (cd4) was calculated. We used the formula Delta = 100(cd - pd)/d 12 times per metaphase, compared each pd vs. each cd, and tested the differences by the Wilcoxon matched-pair test. Although, in the whole sample there were not significant differences respect to cd1, this distance emerged as the proper control. In the eight familial translocations, the three pd vs. cd1 comparisons revealed that in 19/24 times the pd was smaller but only once reached significance (cd1 vs. pd2 in t[3;4]). In the analysis per individual the pd was smaller than cd1 in 19 (pd1), 22 (pd2) and 22 (pd3) cases although only twice reached significance. We conclude that in some translocations, the derivative chromosomes actually lie close from each other or from a normal non-homologous counterpart.