A case of human chimerism detected by unbalanced chromosomal translocation.

Chimerism in humans is usually found only because of discrepancies in unique blood group typing or sex chromosome complements. We describe a case found because of an inherited chromosomal translocation. A female carrier of the balanced reciprocal translocation t(14;20)(q31;q13.3) had a twin pregnancy. After birth the B-twin, a girl, was found to have the balanced translocation. The A-twin, a severely malformed and stillborn boy, had two different karyotypes; a normal 46,XY and an unbalanced translocation derivative 46,XY,-14, +der(14)t(14;20)(q31;q13.3). He was a dispermic chimera, formed by two fertilized oocytes.