Trisomy 21 with 47,+18 lymphocyte cell line: double mitotic nondisjunction

A patient with Down's syndrome was found to have 47,XX,+18/47,XX,+21 mosaicism. Chromosome 18 trisomy was found only in 18% of lymphocytes and not in skin fibroblasts. A likely interpretation is double nondisjunction in a single lymphocyte precursor of a trisomy 21 embryo. A brief review of other cases of mitotic multiple nondisjunction and double aneuploid mosiacism is presented.