SLIGHT INSTABILITY OF A FMR-1 ALLELE OVER 3 GENERATIONS IN A FAMILY FROM THE GENERAL-POPULATION

We report on a family segregating a FMR-1 allele within the ''grey zon e'' of triplet repeat length (n = 51). The allele showed a 1-unit incr ement when transmitted through a female meiosis and a 1-unit increment when transmitted through a male of the next generation. At the follow ing generation, a pregnant woman had amniocentesis performed. The latt er showed she transmitted the allele unchanged (n = 53) to her male fe tus. This family was not ascertained through an affected subject, and there was no family history of mental retardation. Thus our observatio n reflects the natural history of an unstable allele in the general po pulation. Systematic analysis of such alleles may help refine our unde rstanding of the grey zone of triple repeat length. (C) 1996 Wiley-Lis s, Inc.