Molecular characterization of 21p- variant chromosome.

Fortuitously, within a 1-month period, we were referred two individuals for routine cytogenetic amniocenteses involving one chromosome 21 from each patient, which had apparently lost the entire short arm and a major portion of the centromeric alphoid sequences in their amniocytes. Breakage may have occurred within alphoid sequences resulting in extreme variants. Variations of a similar nature were originally referred to as Christchurch (Ch1) chromosomes and have been wrongly determined to be abnormal. The 21p- chromosome variants were similar in both cases, though they are from unrelated individuals. These rare variants, whose origins were both maternal and have no clinical consequences, were characterized by the FISH-technique to provide a greater degree of certainty.