Presumed trisomy for the short arm of chromosome No. 9 not due to inherited translocation.

4 patients trisomic for the short arm of a chromosome No. 9 due to segregation of paternal translocations were described recently by Rethore and associates. In this context, we report on a retarded child, in whom the examination of lymphocyte and fibroblast metaphases revealed the existence of an additional small acrocentric marker chromosome. Cytogenetic data do not exclude the possibility of the marker to be derived from a chromosome No. 9 which exhibits, in the father, a more conspicuous secondary constriction than usual. A breakage event at this site and subsequent non-disjunction of the centric fragment (i.e. chromosome 9 short arm) during paternal gametogenesis could have given rise to the child's chromosomal constitution.