Mosaicism for a small marker chromosome resulting from a familial Robertsonian translocation (21;22).

A mosaic marker chromosome found in amniotic fluid was shown to have originated from the proximal part of the long arm of chromosome 22. This marker is unusual because it is the result of a deletion of a maternally inherited Robertsonian 21;22 translocation. It is suggested that the deletion and marker formation probably occurred post zygotically in the fetus. This rare case illustrates the difficulty in estimating risk of fetal abnormalities associated with de novo marker chromosomes. In this example, although the 'extra' marker chromosome contains euchromatin, the karyotype may still be 'balanced'.