Diploid/triploid mosaicism in dysmorphic patients

Diploid/triploid mosaicism is a dysmorphology syndrome consisting of mental retardation, truncal obesity, body and/or facial asymmetry, growth retardation, hypotonia, a small phallus, malformed low-set ears and micrognathia. In 75% of the cases, the blood karyotype is normal and the diagnosis can only be established after analysis of cultured fibroblasts. This chromosome abnormality may therefore be underdiagnosed. This paper focuses on the identification of mentally retarded and dysmorphic patients with diploid/triploid mosaicism. Detailed clinical description of well-defined patients may help in deciding if a skin biopsy for karyotyping of fibroblasts should be taken. Three new cases are presented, in which DNA marker analysis showed that the extra set of chromosomes in each case was derived from the mother. We present a review of 25 cases described in the literature and we discuss the inclusion of a second polar body into an early diploid embryo as the most likely mechanism.