The effect of Robertsonian translocation on recombination on chromosome 21.

To test the hypothesis that Robertsonian translocation may lead to altered crossing-over on both chromosomes involved in the rearrangement (intrachromosomal effects) and other chromosomes in the cell (interchromosomal effects), we initiated this pilot study utilizing molecular markers (RFLPs) to determine the frequency and approximate location of crossovers on chromosomes 21 of human Robertsonian translocation carriers. Analysis of intrachromosomal effects in five families with Robertsonian translocations involving a chromosome 21 demonstrated an elevation in the amount of crossing-over on chromosomes 21 of the female translocation parent. Several of the crossovers were localized proximal to 21q21.2, suggesting that Robertsonian translocations may lead to an alteration of both the frequency and location of crossing-over. In an assessment of interchromosomal effects in five additional families with a non-21 Robertsonian translocation, no effect could be demonstrated on chromosome 21. The initial data imply that Robertsonian translocation influences the number and position of exchanges on chromosomes 21 involved in the rearrangement, which may be associated with an increased tendency for nondisjunction due to prolonged synapsis of 21. This pilot study demonstrates the utility of this approach in the assessment of intra- and interchromosomal effects of Robertsonian translocations on recombination.