Identification of the origin of a 22p+ chromosome by triplex dosage effect of LDH B, GAPHD, TPI and ENO2.

A 33p+ chromosome in the lymphocytes of an infant with mental deficiency and congenital malformations was found to be a de novo translocation that could not be characterized by banding methods. The demonstration of a dosage effect for four enzymes--TPI, GAPHD, LDHB, and ENO2--in the infant's erythrocytes was consistent with trisomy 12p. The observation demonstrates the usefulness of information provided by the human gene map in the characterization of small chromosome imbalances which defy accurate identification by available banding techniques.