SummaryThe genealogical, cytogenetical and blood‐group data available for six families with an inherited translocation between a chromosome of the 13–15 group and chromosome no. 21 are presented. In two families the translocation was transmitted through three generations, in two families it occurred de novo in the carrier parent, and in two families it could not be decided if the translocation was inherited from a further generation or originated in the gametogenesis of generation I. In five families the transmission to a patient with Down's syndrome was maternal, in one family it was paternal. Meiotic studies of a testicular biopsy from a male carrier, the father of a child with Down's syndrome, revealed trivalent formation in diakinesis.