A survey of a large number of human sera with the heavy chain genetic markers of the gamma-globulin system has revealed an unusual gene complex which is inherited as a unit through two different families. The gene complex involves two pairs of gammaG1 genetic markers which ordinarily behave as homoalleles, Gm(z) and Gm(f) for the Fd part of gammaG1 molecules, and Gm(a) and non-a for the Fc part. Isolation of the gammaG1 fraction from the unusual sera demonstrated the presence of the important non-a antigen in the gammaG1 fraction. Through the use of immunoadsorbents it was shown that these antigens were not part of a single molecule but that separate molecules were involved. The accumulated evidence indicated that the appearance of such homoalleles on the same chromosome probably resulted from a recent gene duplication, giving rise to two gammaG1 cistrons on one chromosome.