Determining the origins and the structural aberrations of small marker chromosomes in two cases of 45,X/46,X, +mar by use of chromosome‐specific DNA probes

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Abstract
A 17‐year‐old girl (S.M.) and a 13‐year‐old girl (C.L.) both with Ullrich‐Turner syndrome (UTS) were found to have 45,X/46,X, +mar mosaicism. The marker chromosomes in both patients were very small in size. In S.M. the marker chromosome was present in 80% of phytohemaglutinin‐stimulated lymphocytes, 28% of skin fibroblasts, and 11–20% of gonadal fibroblasts. In C.L., the small marker chromosome was found in 50% of stimulated lymphocytes. S.M. is of normal height, but C.L. is short. Molecular hybridization with a number of Y‐specific DNA probes demonstrated their presence in S.M. but absence in C.L. In situ hybridization with Y‐specific and X‐centromere‐specific DNA probes confirmed the Y origin of the marker chromosome in S.M. and the X origin of the minute chromosome in C.L. Biotinylated centromere and telomere probes were also used for in situ hybridization to show the presence of centromeric and telomeric sequences in the Y‐marker chromosome, suggesting that the deletion of this marker chromosome is interstitial.