A Defective Extra Chromosome Associated with Clinicial 17-18 Trisomy Syndrome

A male infant was found to have many clinical features typical of 17-18 trisomy. However he had an unusual chromosome defect which has not previously been reported, an additional medium chromosome with arms of equal length. Its structure is compatible with that of a 17-18 long-arm isochromosome. His clinical features and postmortem findings are compared to those associated with usual 17-18 trisomy, and support the thesis that he possessed a defective additional 17 or 18 chromosome. Reports of such defective extra chromosomes are rare. Additional studies are needed to confirm or to disprove an association between such clinical and chromosome defects.