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 origin ated from the proximal part of the long arm of chromosome 22. This marker i s unusual because it is the result of a deletion of a maternally inherited Robertsonian 21;22 translocation, It is suggested that the deletion and mar ker formation probably occurred post zygotically in the fetus. This rare ca se illustrates the difficulty in estimating risk of fetal abnormalities ass ociated with de novo marker chromosomes. In this example, although the 'ext ra' marker chromosome contains euchromatin, the karyotype may still be 'bal anced'.