Structural chromosome abnormalities in Down syndrome: a study of two families

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Abstract
Two families were ascertained through a proband with Down syndrome and a structural rearrangement involving two chromosomes 21. It is suggested that in one patient the chromosome is an isochromosome formed by misdivision of the centromere of a maternal telocentric chromosome 21 and that in the other a Robertsonian translocation involving chromosome 21 was inherited from the mother, who is a 46, XX/46, XX,–21,+t(21q21q) mosaic. The origin of the mosaicism is discussed and considered to be likely to be the result of breakage and reunion at the chromatid, rather than the chromosome, level.