Unusual segregation of constitutional 11q;22q translocation may be explained by crossover in interchange segment, followed by 3 : 1 segregation at meiosis I

Recently, Lockwood et al. (1989) reported an interesting and unusual segregation pattern in a family with the common reciprocal translocation t(11; 22)(q23.3; q11.2). The proband, with the phenotype normally associated with the presence of the derived chromosome 22, had acquired two copies of the derived 22 and one copy of the derived 11 from the father, who carried the balanced form of the translocation. It was suggested that this pattern of segregation resulted either from nondisjunction in paternal meiosis II, or from postzygotic nondisjunction. However, there is a third way in which the observed segregation may have occurred. If a crossover involving the derived and the normal chromosome 22, between the centromere and the translocation breakpoint (interchange segment), was associated with 3:1 segregation at meiosis I such that the derived and normal chromosome 22 and the derived 11 passed together into meiosis II, this could have led to the observed karyotype in the proband.