The ratio of de novo unbalanced translocation to 47, trisomy 21 Down syndrome. A new method for human mutation surveillance and an apparent recent change in mutation rate resulting in human interchange trisomies in one jurisdiction.

The Down syndrome phenotype may be associated with, among other genotypes, an unbalanced Robertsonian translocation producing an "interchange trisomy" with 46 chromosomes, or 47, trisomy 21. Translocations, like specificlocus point mutations, result from a direct change in structural chromosome elements. In contrast 47, trisomy 21 results from meiotic non-disjunction. Mutation rates for interchange trisomies may be followed indirectly by determining the ratio of instances of Down syndrome associated with a new translocation mutation to those produced by 47, trisomy 21, which accounts for the bulk of the Down syndrome phenotype. This genotypic ratio can be analyzed in data from cytogenetic laboratories, clinics, and chromosome registries and does not depend upon intensive chromosome screening of newborn populations. A similar approach can be adopted to follow trends in Patau syndrome. The genotypic ratio, stratified by maternal age, may in addition, provide a sentinel index for changes in human specific-locus mutations and perhaps other adverse health consequences. Analysis of data from the New York State-North-eastern chromosome registry revealed a two- to three-fold increase in the genotypic ratio for both Down syndrome and Patau syndrome for individuals born in 1973, 1974 and 1975 compared to those born in earlier years.