Robertsonian translocations are characterized by centric fusion of two non-homologous acro- or telocentric chromosomes. Such translocations can be maintained in populations if in the chromosomally heterozygous individuals meiosis takes place regularly. In this case the question arises as to the existence of secondary isolating mechanisms that inhibit genetic exchange between the translocation chromosomes and their non-fused counterparts. The present paper treats this question by analyzing simultaneous changes at two gene loci, one located on each of the two constituent chromosomes. The analytic results are discussed in terms of opportunities for genetic differentiation to occur within a population polymorphic for Robertsonian translocations and are opposed to current theories on this subject.