A family is described in which the mother has an 18p-chromosome, one normal 18, and a probable i(18p). One of the daughters of this woman inherited the 18p-chromosome, and her phenotype resembles that of other 18p-cases. The other daughter inherited the presumed i(18p) chromosome, and her phenotype resembles that of some cases with extra, small metacentric chromosomes. The clinical, chromosomal, and familial evidence suggest that these abnormal chromosomes originated in the occurrence of one transverse break of the centromere and subsequent misdivision of a chromosome 18 in an earlier generation of this family. According to this interpretation, the mother is trisomic for 18p, one daughter is monosomic and the other daughter tetrasomic for this chromosomal region.
Two sibs show a strikingly concordant syndrome of congenital anomalies and G-banding reveals that each has partial trisomy 20p resulting from a t(18;20) translocation. They resemble other cases of partial trisomy 20p in some respects but also differ in some ways. Their normal sib, mother, and half-aunt are balanced heterozygotes for the t(18;20) translocation. The segregation of the balanced translocation in this family is associated with an extremely poor reproductive record. The segregation pattern closely parallels that of a t(13;20) translocation in a family described by Carrel et al. (1971) and Francke (1972). The similarity of segregation patterns is predictable on the basis of probable pachytene configurations, but the dissimilarity of phenotypes between families is not readily explained.
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