Homodicentric chromosomes: a distinctive type of dicentric chromosome.

This report describes two patients with a distinctive type of dicentric autosomal chromosome formed by breakage and union between homologous chromosomes. These stable chromosomes possess two C bands, implying the presence of two centromeric regions. The first child, evaluated for dysmorphic features was shown to have an abnormal chromosome 16, designated as 46, XX, -16, + dic (16) (pter leads to cen leads to q22::p11 leads to qter). The second case is a child with the typical features of trisomy 18 whose karyotype is designated as 46, XX, -18, + dic (18) (qter leads to p11.1 :: p11.3 leads to cen leads to qter). The stability of these chromosomes is presumably in result of centromere suppression and associated premature centromere division of the suppressed centromere. The possible mechanism of formation of these homodicentric chromosomes is presented, and a comparison is made between them and three patients with dicentric X chromosomes.