Isolated skeletal malformations in a child with a small mosaic ring microduplication of 18 p11.21q11.2: genotype-phenotype correlations.

We describe a developmentally normal Amish child who has a karyotype with 47 chromosomes, including a supernumerary ring-shaped chromosome 18 in each metaphase studied. The only phenotypic findings in the patient were hemivertebrae and rib anomalies. Further analysis of interphase cells revealed an additional, less frequent mosaic, apparently normal cell population. Genes in the triplicated region that possibly are contributing to her skeletal phenotype include GATA6, MC2R, MC5R, RBBP8, ESCO1, and ROCK1, among others. By studying such patients with abnormal genetic dosage, genotype-phenotype correlations can be used to refine gene function.