genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar wor ...

, pseudolinkage is a characteristic of a

heterozygote Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...

for a

reciprocal translocation In genetics, chromosome translocation is a phenomenon that results in unusual rearrangement of chromosomes. This includes balanced and unbalanced translocation, with two main types: reciprocal-, and Robertsonian translocation. Reciprocal translo ...

, in which genes located near the

translocation Translocation may refer to: * Chromosomal translocation, a chromosome abnormality caused by rearrangement of parts ** Robertsonian translocation, a chromosomal rearrangement in pairs 13, 14, 15, 21, and 22 ** Nonreciprocal translocation, transfer ...

breakpoint behave as if they are linked even though they originated o
nonhomologous chromosomes

Linkage Linkage may refer to: * ''Linkage'' (album), by J-pop singer Mami Kawada, released in 2010 *Linkage (graph theory), the maximum min-degree of any of its subgraphs *Linkage (horse), an American Thoroughbred racehorse * Linkage (hierarchical cluster ...

is the proximity of two or more markers on a

chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

; the closer together the markers are, the lower the probability that they will be separated by recombination.

Genes In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...

are said to be linked when the frequency of

parental A parent is a caregiver of the offspring in their own species. In humans, a parent is the caretaker of a child (where "child" refers to offspring, not necessarily age). A ''biological parent'' is a person whose gamete resulted in a child, a male t ...

type progeny exceeds that of recombinant progeny.

Not occur in translocation homozygote

During

meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...

in a translocation homozygote, chromosomes segregate normally according to Mendelian principles. Even though the genes have been rearranged during

crossover Crossover may refer to: Entertainment Albums and songs * ''Cross Over'' (Dan Peek album) * ''Crossover'' (Dirty Rotten Imbeciles album), 1987 * ''Crossover'' (Intrigue album) * ''Crossover'' (Hitomi Shimatani album) * ''Crossover'' (Yoshino ...

, both

haploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...

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

In translocation heterozygote, however, certain patterns of chromosome segregation during meiosis produce genetically unbalanced gametes that at

fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...

become deleterious to the

zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicellula ...

. 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 Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately res ...

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 Synapsis is the pairing of two chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I of meiosis. Whe ...

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 The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers a ...

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 Spindle may refer to: Textiles and manufacturing * Spindle (textiles), a straight spike to spin fibers into yarn * Spindle (tool), a rotating axis of a machine tool Biology * Common spindle and other species of shrubs and trees in genus ''Euony ...

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 In cell biology, the spindle apparatus refers to the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter Cell (biology), cells. It is referred to as the mitotic spindle durin ...

, this normal disjunction of homologous produces one of two equally likely patterns of segregation.

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

duplication Duplication, duplicate, and duplicator may refer to: Biology and genetics * Gene duplication, a process which can result in free mutation * Chromosomal duplication, which can cause Bloom and Rett syndrome * Polyploidy, a phenomenon also known ...

(of the region found in both the normal and the translocated chromosome in that gamete) and a correspondingly large

deletion Deletion or delete may refer to: Computing * File deletion, a way of removing a file from a computer's file system * Code cleanup, a way of removing unnecessary variables, data structures, cookies, and temporary files in a programming language * ...

(of the region found in neither of the chromosomes in that gametes), which make them genetically unbalanced. Zygotes formed by union of these gametes with a normal gametes are usually not viable.

Adjacent-2 segregation pattern

Because of the unusual cruciform pairing configuration in translocation heterozygotes,

nondisjunction Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division (mitosis/meiosis). There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I ...

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 th
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

* {{refend Genetics