MOSAICISM WITH A NORMAL-CELL LINE AND AN AUTOSOMAL STRUCTURAL REARRANGEMENT

Over three decades, 12 cases of mosaicism for an autosomal rearrangeme nt were recognised in the major cytogenetics laboratories in New Zeala nd, eight of which were studied between 1990 and 1992. One case infere ntially involved the gonad, eight the soma, and three both gonad and s oma. This mosaicism could have arisen as a postzygotic event either in a conceptus that was initially normal, with the generation of an abno rmal cell line, or in a conceptus having a supernumerary chromosome wh ich was lost at a subsequent mitosis, thereby restoring a normal cell line. Three of the 12 cases involved a presumed direct duplication, an otherwise very uncommon rearrangement. This may indicate a propensity for direct duplications to arise at mitosis rather than at meiosis; u nequal sister chromatid exchange is mechanism. Mosaicism relevance for genetic counselling, as an intragonadal cell line carrying a rearrang ement could generate multiple unbalanced gametes. Mosaicism for an aut osomal rearrangement may be very much more common than is, or ever cou ld be, recognised.