Translocations

Non-homologous chromosomes exchange segments with each other resulting in aberrations. These aberrations are known as translocations. Ectopic recombination also causes translocation mutations. Though translocations contribute to species evolution, they mainly affect the genotype of the individuals leading to a genetic disease. Translocation homozygotes and heterozygotes decide the type of translocation mutation. Translocations not only change the position of the segments but also change the gene sequence. Though there is a change in the sequence, there is no loss or gain of the genetic material. There are two key types of translocations such as non-reciprocal interchromosomal and intrachromosomal translocations. Translocation occurring within a chromosome is known as nonreciprocal intrachromosomal translocation. Translocations occurring between the two chromosomes are known as interchromosomal translocations. Translocation mutations arise due to the improper repair of double-strand breaks. Chromosomal translocations also lead to cancers.

                                             Image: Types of translocation mutations

Unbalanced gamete production is a common feature during meiosis involving translocation mutations. The normal pairing of the chromatids occurs in homozygotes during meiosis. Hence, crossing over does not produce any abnormal chromatid structures. The heterozygotes involve chromatids trying best to pair properly. However, they involve normal and translocated chromosomes resulting in cross-like structures during prophase I of the meiosis.  Different types of segregation patterns get involved during anaphase I. Alternate segregation involves the migration of the alternate centromeres to the same pole resulting in the formation of two gametes. Each gametic product arising due to alternate segregation consists of a complete set of genes. One gamete consists of normal chromosomes and the other gamete consists of the translocated chromosomes. Migration of adjacent non-homologous centromeres towards the same pole depicts adjacent-1 segregation. Both the gametes contain gene duplication and deletion. Different parts of the adjacent homologous centromeres migrate to the same pole in adjacent-2 segregation. Inviable products with gene duplications and deletions arise due to this type of segregation.

Balanced translocations do not lead to net loss or gain of the genetic material. They exhibit full functionality. Unbalanced translocations lead to extra or missed chromosomes and do not show complete functionality. Balanced translocations without involving CNVs do not largely affect the phenotypic factors. However, the translocation carriers possess a high risk of infertility, miscarriage, and unbalanced fetus. Both the reciprocal and Robertsonian translocations may involve the balanced or unbalanced type of translocations. Balanced translocations resulting in Robertsonian translocations involve pairing between the translocated chromosome and the homologous chromosomes during meiosis resulting in the formation of a trivalent. Balanced translocations involving reciprocal translocation result in the formation of tetravalent. The detection of cryptic balanced translocations through karyotyping is difficult. Hence, whole genome sequencing contributes to detecting the same.

Image 2: Reciprocal and Robertsonian translocations

Reciprocal translocations:

A reciprocal translocation involves two reciprocally exchanged parts of segments of the chromosomes. We have already discussed the homozygotes and heterozygotes involving the reciprocal translocations. A condition known as semisterility is common in heterozygotes involving reciprocal translocation. Only half of the offspring exhibit normal characteristics. There arises a synapsis between the normal and the translocated chromosome in a heterozygote. Crossing over takes place between the non-sister chromatids in the homologous chromosome arms. The adjacent-1 segregation is also known as disjunctional segregation. It involves deficiency of the distal part of the translocated chromosome and large duplications. The adjacent-2 segregation is also known as non-disjunctional segregation. Since the homologous centromeres migrate towards the same pole, it leads to the deficiency of the proximal part of the translocated chromosomes and large duplications. The adjacent-1 and adjacent-2 segregations increase the chances of lethality.

Robertsonian translocation:

It is a type of non-reciprocal translocation. Robertsonian translocations result in the formation of a chromosome occurring due to the fusion of two non-homologous acrocentric chromosomes. The American scientist known as Rees Brebner Robertson first studied and explained translocation in grasshoppers. Hence, these translocations are known as Robertsonian translocations. The synonyms for the Robertsonian translocations include whole-arm translocations or centric fusion translocations. Mostly, it involves the fusion of long arms. Chromosome pairs such as 13, 14, 15, 21, and 22 generally undergo Robertsonian translocation. Robertsonian translocations either involve homologous or non-homologous chromosomes. Robertsonian translocation carriers are usually healthy. However, such individuals pass on the translocation to the next generation based on their partners. Robertsonian translocations result in infertility and rare disorders.

·        Trisomy and Monosomy due to Robertsonian translocation:

The 14:21 type of Robertsonian translocation is common. Down’s syndrome is a typical example of trisomy 21. The fetus consists of three copies of the 21^st chromosome instead of two copies. Robertsonian translocation leads to familial down’s syndrome with a high risk of inheritance. Monosomies also occur due to this type of translocations. Monosomy 21 and monosomy 14 commonly arise due to 14:21 Robertsonian translocations.

Translocations in Oenothera plant:

Oenothera consists of a very large number of herbaceous plants. The common name of Oenothera species is known as evening primrose plant. These plants have a total of 145 species and are known as Angiosperms. Chromosomes in the Oenothera species show the ability to form circles instead of pairs during meiosis. These plants follow non-Mendelian patterns. The plants also display the hybrid vigor of heterosis. These plants exhibit reciprocal translocations. Oenothera species follow alternate segregation. Hence, they escape the semisterility. The translocation heterozygotes result in the formation of ring chromosomes. The karyotype of the complex translocation heterozygote gets restored.

References:
[1]  Chromosome Translocation, Yu Zhang
[2]  IGenetics, Peter Russell, second edition

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