PRENATAL DETECTION OF SHORT ARM DELETION AND ISOCHROMOSOME-18 FORMATION INVESTIGATED BY MOLECULAR TECHNIQUES

A patient was referred for amniocentesis because of advanced maternal age and polyhydramnios. The fetal karyotype was a mosaic 46,XX,del(18) (p11.1)/46,XX,-18, +i(18q)de novo. The deletion appeared to encompass the whole short arm as evidenced by G banding and in situ hybridisatio n. However, telomere sequences were found on both ends of the deleted chromosome as well as the isochromosome. The normal 18 and the isochro mosome showed more alphoid sequences than the del(18). Subsequent pass ages of the cell lines showed an increase in the frequency of the isoc hromosome from 20% to about 30%. Possible mechanisms are discussed.