A patient with Down’s syndrome was found to have a modal number of 46 with a trisomy 21 and a reciprocal translocation in the 13–15 group. Several members of his immediate family were phenotypically normal carriers of a similar reciprocal translocation in the 13–15 group. Extensive blood grouping studies gave the very tentative suggestion of a possible linkage between the Gm-locus and the translocation chrom
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