Robertsonian translocation and an extra microchromosome: aetiology and effect on meiotic segregation.

A familial Robertsonian translocation accompanied by a marker chromosome t(14q;21q), + iso dic (?14?21)(pter----q11.1) is described. The evolution of the marker is considered in relation to a model proposed by ultrastructural studies of acrocentric bivalents for the formation of Robertsonian-type translocations. Previous reports have suggested that a marker chromosome may be of pathogenetic importance for non-disjunction of chromosome 21. The effect of the marker chromosome on meiotic segregation in the family described and its implications upon genetic counselling are considered.