Isochromosome 18p in a mother and her child.

We present a familial case of isochromosome 18p [i(18p)] as a supernumerary chromosome. The mother, who is a mosaic for i(18p) with partial tetrasomy 18p syndrome, transmitted the isochromosome to her only child. The child has the full syndrome of tetrasomy 18p. This is the first case of mosaicism i(18p) in an adult patient with clinical manifestations.