Partial trisomy of chromosome 18 (pter----q12) following a familial 18;21 translocation rcp(18;21)(q12;q11).

A 1-year-old boy with trisomy 18 (pter----q12) following a paternal balanced translocation revealed microcephaly, a pattern of minor dysmorphic features including upslanting narrow palpebral fissures, receding forehead, large nose and receding mandible, cryptorchidism, flexion contractures of fingers, a cardiac malformation and moderate mental retardation. While pure trisomy 18p generally goes along with a near-normal phenotype, additional trisomy of only a short segment of the proximal long arm 18 has a distinct negative influence on the phenotype, as seen in our proband.