Grandmaternal origin of an isochromosome 18p present in two maternal half-sisters

The syndrome of tetrasomy 18p has been well documented in the literature. T his is typically a result of a supernumerary isochromosome 18p, that has ar isen during maternal meiosis II. This report presents clinical and molecula r findings in two maternal half sisters with an isochromosome 18p, The isoc hromosome is inferred to have arisen during meiosis in the maternal grandmo ther and to have undergone mitotic and meiotic recombination in the mother of JJ and AT. The abnormal cell line may be restricted to the gonad in the mother as only normal 46,XX cells were detected by cytogenetic analysis of her blood or fibroblasts and physical examination revealed only normal find ings, Thus, the isochromosome, although present at fertilization, must have been lost from the majority of embryonic precursor cells, This case raises important genetic counseling issues concerning recurrence risks. (C) 2001 Wiley-Liss, Inc.