[Cytogenetic study of the parents of 85 index cases with regular trisomy 21].

The parents of 85 Down syndrome cases with regular 21 trisomy were studied cytogenetically. The lymphocyte chromosomes were stained with G-banding technique and 100 metaphases were analyzed in the father, the mother and the index case. Among the mothers two cases of mosaic 46, XX/47, XX, +21 (2.35%) were found and among the fathers no mosaic cases were found. Reciprocal and robertsonian translocations were not observed. In one of the fathers a pericentric inversion of the Y chromosome (0.61%) was found. The results are compared with those of similar studies and discussed in relation to genetic counseling and to the possible existence of an "interchromosomal effect" in man.