The karyotype of a child with dysmorphic findings suggestive of both trisomy 13 and the 13q-syndrome was found to have cells with one of two different dicentric chromosomes: one bearing a duplication of chromosome 13q [46,XX,-13, + psu dic(13)t(13;13)(pter→ce→q34::q34→pter)] and the other a deletion of 13q [46,XX,-13,+psu dic(13)t(13;13)(pter→cen→q22::q11→pter). Longitudinal cytogenetic studies in leukocytes demonstrated a loss of those cells possessing the small dicentric [psu dic(13)(q22;q11)], whereas fibroblasts from two separate skin biopsies contained only this marker. Q-band polymorphisms indicated that both dicentrics were of paternal origin, with the smaller dicentric derived from the larger via the bridge-breakage-fusion cycle. The presence of two active centromeres could not be confirmed in either dicentric.
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