Discordant Phenotype in Monozygotic Twins with Mosaic Trisomy 12p in Lymphocytes

The study reported on monochorionic diamniotic male twins with discordant phenotypes due to mosaic trisomy 12p. Both twins had approximately 50% of lymphocytes with trisomy 12p, but the affected twin exhibited high-grade mosaicism in buccal smear (77%) and hair follicles (85%), while the unaffected twin showed no additional 12p chromosome fragment in these tissues. Instead, the unaffected twin had a minute supernumerary marker chromosome in 3% of buccal smear and hair follicle cells. The discordant phenotypes were likely due to prenatal twin-to-twin transfusion, leading to an unequal distribution of aneuploid cells in different tissues. This case highlighted the inadequacy of lymphocyte analysis alone for detecting mosaicism in monozygotic twins.