Turner phenotype in a girl with a 45,X/46,XX/47,XX,+18 mosaicism.

We report a girl with Turner syndrome phenotype, whose karyotype on amniocyte culture was 45,X, while cytogenetic analysis on peripheral blood lymphocytes showed the presence of a mosaic chromosome constitution with three different cell lines: 45,X[5]/46,XX[3]/47,XX,+18 [35]. No signs of trisomy 18 were observed and a follow up during childhood revealed normal psychomotor development. Parental origin and mechanism of formation were studied using high polymorphic microsatellites and Quantitative Fluorescent PCR. The 18-trisomic cells showed one paternal allele and two maternal homozygous alleles at different loci of chromosome 18, suggesting a maternal M-II meiotic or a postzygotic error. A biparental origin of the X-alleles in the trisomic cells were determined, being the paternal allele retained in the 45,X cells. The possible mechanism of formation implying meiotic and/or mitotic errors is discussed.