Trisomy/partial monosomy 13 mosaicism associated with relatively mild clinical malformation

A female fetus with a rare de novo chromosome abnormality involving mosaicism with two cell lines with trisomy 13 and presumptive partial monosomy 13 due to a Robertsonian translocation and ring chromosome, respectively, was prenatally diagnosed. On termination this case, together with apparently the only other three reported cytogenetically similar cases, was found to have much less severe clinical abnormalities than might have been predicted. The possibility in these cases that the effects of the trisomy and monosomy for the common segment of chromosome 13 may have counterbalanced each other at the tissue level in embryogenesis thereby resulting in less disturbance of morphogenesis than would be expected for each type of imbalance acting in isolation is considered.