THE EFFECT OF ROBERTSONIAN TRANSLOCATION ON RECOMBINATION ON CHROMOSOME-21

To test the hypothesis that Robertsonian translocation may lead to alt ered crossing-over on both chromosomes involved in the rearrangement ( intrachromosomal effects) and other chromosomes in the cell (interchro mosomal effects), we initiated this pilot study utilizing molecular ma rkers (RFLPs) to determine the frequency and approximate location of c rossovers on chromosomes 21 of human Robertsonian translocation carrie rs. Analysis of intrachromosomal effects in five families with Roberts onian translocations involving a chromosome 21 demonstrated an elevati on in the amount of crossing-over on chromosomes 21 of the female tran slocation parent. Several of the crossovers were localized proximal to 21q21.2, suggesting that Robertsonian translocations may lead to an a lteration of both the frequency and location of crossing-over. In an a ssessment 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 translocati on influences the number and position of exchanges on chromosomes 21 i nvolved in the rearrangement, which may be associated with an increase d tendency for nondisjunction due to prolonged synapsis of 21. This pi lot study demonstrates the utility of this approach in the assessment of intra- and interchromosomal effects of Robertsonian translocations on recombination.