An individual or cell in which two pairs of homologous chromosomes have reciprocally exchanged nonhomologous segments between one member of each pair. Each chromosome pair contains both homologous and nonhomologous segments, i.e., one normal (untranslocated chromosome) and one translocated chromosome. A translocation heterozygote (illustrated below) forms a quadrivalent chromosomal association during pachynema (see meiosis), and the subsequent segregation of the four chromosomes is determined by their centromere orientations. In the case of adjacent disjunction, (or adjacent segregation), each daughter cell receives a normal and a translocated chromosome. The gametes produced from such cells are inviable because they contain certain genes in duplicate and are deficient for others. Two kinds of adjacent disjunction are recognized. In adjacent-1 segregation, homologous centromeres go to opposite anaphase poles. In adjacent-2 segregation, homologous centromeres go to the same anaphase pole. In the case of alternate disjunction, both translocated chromosomes go to one cell and both normal ones to the other. The gametes that result are viable because they contain all the genetic material.
Subjects: Genetics and Genomics.