Pseudolinkage
In
Not occur in translocation homozygote
During
In translocation heterozygote
In translocation heterozygote, however, certain patterns of chromosome segregation during meiosis produce genetically unbalanced
Alternate segregation pattern
In the alternate segregation pattern, the two translocation chromosomes (T1 and T2) go to one pole, while the two normal chromosomes (N1 and N2) move to the opposite pole. Both kinds of gametes resulting from this segregation (T1, T2, and N1, N2) carry the correct haploid number of genes; and the zygotes formed by union of these gametes with normal gamete will be viable.
Adjacent-1 segregation pattern
In the adjacent-1 segregation pattern, homologous centromeres disjoin so that T1 and N2 go to one pole, while the N1 and T2 go to the opposite pole. Consequently, each gamete contains a large
Adjacent-2 segregation pattern
Because of the unusual cruciform pairing configuration in translocation heterozygotes, nondisjunction of homologous centromeres occurs at a measurable but low rate. This nondisjunction produces an adjacent-2 segregation pattern in which the homologous centromeres N1 and T1 go to the same spindle pole while the homologous centromeres N2 and T2 go to the other spindle pole. The resulting genetic imbalances are lethal after fertilization to the zygotes containing them.
Thus, in a translocation heterozygote, only the alternate segregation pattern yields viable progeny in outcrosses, the equally likely adjacent-1 pattern and the rare adjacent-2 pattern do not. Because of this, genes near the translocation breakpoints on the nonhomologous chromosomes participating in a reciprocal translocation exhibit pseudolinkage: They behave as if they are linked.
References
- Hartwell L, Hood L, Goldberg ML, Reynolds AE, Silver LM, Veres R (2004). Genetics: from genes to genomes (Second ed.). Boston: McGraw-Hill Higher Education. ISBN 0-07-291930-2.