We report here on 3 familial whole-arm translocations (WATs), namely the 8th instance of t(l;19)(p10;q10) and 2 novel exchanges: t(9;13)(p10;q10) and t(12;21)(p10;q10). The exchanges (1;19) and (12;21) were ascertained through a balanced carrier, whereas the t(9;13) was first diagnosed in a boy with atrisomy 9p syndrome and der(9p13p). Results of FISH analyses with the appropriate a-satellite probes were as follows. Family 1, t(l;19): the D1Z5 probe gave a strong signal on both the normal chromosome 1 and the der(1q19p) as well as a weak signal on the der(1p19q). Family 2, t(9;13): the centromere-9 alphoid and D13Z1 /D21Z1 probes under standard stringency gave no signal on the der(9p 13p) in both the proband and a carrier brother, whereas the der(9ql3q) was labelled only with the centromere-9 alphoid repeat in the latter; yet, this probe under low stringency revealed a residual amount of alphoid DNA on the der(9p 13p) in the carrier. Family 3, t(12;21): the D12Z3 probe gave a signal on the normal chromosome 12 and the der(12p21q), whereas the D13Z1/D21Z1 repeat labelled the der(12q21p), the normal chromosome 21, and both chromosomes 13. Out of 101 WATs compiled here, 73 are distinct exchanges, including 32 instances between chromosomes with common alphoid repeats. Moreover, 7/9 of recurrent WATs involved chromosomes from the same alphoid family. Thus constitutional WATs appear to recur more frequently than other reciprocal exchanges, often involve chromosomes with common alphoid repeats, and can mostly be accounted for the great homology in alphoid DNA that favours mispairing and illegitimate nonhomologous recombination.
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