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A family showing transmission of a D/D type translocation is reported. The chromosomes involved are probably Nos. 13 and 15 (Denver). Attempts to localize a number of marker genes in relation to the centromere of the translocated chromosome were unsuccessful. The propositus in this family was a case of regular (21-trisomy) Down’s syndrome (Mongolism). The possible association between non-disjunction and the translocation is discussed both in relation to the present family and others in the literature.

An attempt is made to obtain an estimate of the frequency of interchange trisomic Down’s syndrome within the general population. This estimate is used to calculate the interchange mutation rate (τ) involving chromosome 21 and resulting in the formation of interchange trisomics. Inadequacies and shortcomings of the data used, the assumptions required and the method employed are discussed. A maximum estimate of 2.71 × 10^–5 (1 in 37,000) and a minimum of 2.07 × 10^–5 (1 in 48,000) interchanges per chromosome 21, per gamete, per generation, resulting in the formation of interchange trisomics is obtained.

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