Pseudolinkage

In genetics, pseudolinkage is a characteristic of a heterozygote for a reciprocal translocation, in which genes located near the translocation breakpoint behave as if they are linked even though they originated on nonhomologous chromosomes. In genetics, pseudolinkage is a characteristic of a heterozygote for a reciprocal translocation, in which genes located near the translocation breakpoint behave as if they are linked even though they originated on nonhomologous chromosomes. Linkage is the proximity of two or more markers on a chromosome; the closer together the markers are, the lower the probability that they will be separated by recombination. Genes are said to be linked when the frequency of parental type progeny exceeds that of recombinant progeny. During meiosis in a translocation homozygote, chromosomes segregate normally according to Mendelian principles. Even though the genes have been rearranged during crossover, both haploid sets of chromosomes in the individual have the same rearrangement. As a result, all chromosomes will find a single partner with which to pair at meiosis, and there will be no deleterious consequences for the progeny. In translocation heterozygote, however, certain patterns of chromosome segregation during meiosis produce genetically unbalanced gametes that at fertilization become deleterious to the zygote. In a translocation heterozygote, the two haploid sets of chromosomes do not carry the same arrangement of genetic information. As a result, during prophase of the first meiotic division, the translocated chromosomes and their normal homologs assume a crosslike configuration in which four chromosomes, rather than the normal two, pair to achieve a maximum of synapsis between similar regions. We denote the chromosomes carrying translocated material with a T and the chromosomes with a normal order of genes with an N. Chromosomes N1 and T1 have homologous centromeres found in wild type on chromosome 1; N2 and T2 have centromeres found in wild type on chromosome 2.During anaphase of meiosis I, the mechanisms that attach the spindle to the chromosomes in this crosslike configuration still usually ensure the disjunction of homologous centromeres, bringing homologous chromosomes to opposite spindle poles. Depending on the arrangement of the four chromosomes on the metaphase plate, this normal disjunction of homologous produces one of two equally likely patterns of segregation.