Trisomy 13 due to rea(13q;13q) is caused by i(13) and not rob(13;13)(q10;q10) in the majority of cases.

We have used 20 PCR-based DNA polymorphisms to determine whether trisomy 13 due to de novo rea(13q;13q) in six cases is caused by translocation (13q;13q) or isochromosome (13q;13q); to determine the parental origin of the rearrangements and the mechanisms of formation. The six probands were three liveborn children with clinical features characteristic of Patau's syndrome and three fetuses diagnosed prenatally by amniocentesis or CVS. Five cases were isochromosomes with two identical q arms, one of maternal and four of paternal origin. Only one case was a Robertsonian translocation of maternal origin.