Inversion mutation

A chromosomal segment gets excised and reinserted at an angle of 180⁰ from the original one. Paracentric and pericentric inversions are involved in mutations. There is no involvement of the centromere in paracentric inversions. A pericentric inversion includes centromere. Inversion mutation does not lead to the loss of the genetic material. The homozygotes or heterozygotes get affected by inversions. Two breaks in a chromosomal segment followed by a reversal of the segment before healing of the breaks result in the formation of an inverted segment. Inversions also arise due to ectopic recombination. Heterozygotes show the presence of one wild-type chromosome and one chromosome having an inversion.

Heterozygous inversion does not complicate mitosis. However, problems arise during meiosis. Mainly, the homologous recombination between the inverted regions leads to complications. The homologous recombination follows gene for gene pairing especially during the prophase I of meiosis. The gene for gene pairing requires an inverted loop structure. The homologous chromosomes disjoin without any problem if there is no crossing over between the inverted regions in a loop. Four types of gametes arise due to the above condition. Two gametes carry structurally normal chromosomes. The remaining two gametes carry inverted chromosomes.

Image 1: Paracentric and pericentric inversions

Paracentric inversion:

Such inversions do not include the centromere. The chromosomal structural changes totally depend on the crossing over event occurring in the looped region. The inverted loops do not create any hindrances in chromosomal structure and cellular development if there is no crossing over in the inverted regions in the loops. Also, the homologous chromosomes separate normally during the anaphase I. However, the presence of the crossing over the event in inverted loops leads to duplications and deletions. They arise due to the physical joining of the chromatids. The crossing over leads to the formation of products incapable of including in a normal gamete. These products include a dicentric chromosome and a reciprocal product known as an acentric chromosome. The chromosome lacking a centromere, known as an acentric chromosome consists of deleted and duplicated regions. The cell loses the acentric and dicentric chromosomes. Those chromatids not participating in the crossing over become eligible for getting included in the gametes. One of the products consists of inversions. Most of the chromosomes having inverted regions suppress the crossing over process.

Image 2: Paracentric inversion process

Pericentric inversion:

Breaks occurring at each side of the centromere lead to the pericentric inversions. The pairing of the pericentric chromosomes also involves loop formation just like the paracentric inversions. A loop formation occurs during the synapsis including the centromere and usually involves a large inverted segment. It does not involve the formation of a dicentric chromatid. Crossing over gives rise to the chromatids having deletions or duplications. Single or double crossing over occurs during meiosis. Single crossing over leads to two chromatids having deficiency duplications. The gametes carrying the products of single crossovers lead to sterility of the gametes. Two types of chromatids get involved. One type of chromatid will be normal since it is a non-crossover chromatid. The other chromatid will be an inverted one. The double crossovers involving the two same chromatids do not change the products. Pericentric inversions also include three-strand and four-strand double crossovers. Pericentric inversions also lead to polymorphisms.

Image 3: Pericentric inversions 

Other types of inversions:

There are several other types of inversions. They include complex inversions, independent inversions, direct tandem inversions, reverse tandem inversions, included inversions, and overlapping inversions. More than one inversion in a chromosome is known as complex inversion. The independent inversions, direct inversions, reverse tandem inversions, overlapping inversions, and included inversions belong to the class of complex inversions. Different regions involving inversions separated by an uninverted or a normal segment exhibit independent inversions. Those segments directly adjacent to each other not involving a normal segment exhibit direct tandem inversion. Mutually interchanged positions of the inverted segments placed adjacent to each other exhibit reverse tandem inversions. Inversion also occurs in a segment already having an inverted sequence. This type of inversion is known as included inversion. The second inverted segment exhibits a normal sequence. Overlapping inversions involve inversions of a part of a segment of already inverted chromosomes.

Infertility is common among inversion heterozygotes:

Inversion heterozygotes lead to the production of unbalanced gametes. They show the presence of the deficiency-duplication chromatids. Such chromatids arise due to the crossing over between the chromatids in the inversion loops. Both the pericentric and paracentric inversions show different effects on fertility. Also, the fertility issues vary from organism to organism. For example, pericentric inversions in Drosophila and few animals lead to abortion of the zygote. The pericentric inversions in plants lead to variable pollen or ovule sterility. Infertility and abortions in humans also arise due to pericentric inversions. Some of the paracentric inversions involve bridge elimination mechanisms. Complete infertility or partial sterility also arise due to paracentric inversions. Some plants, insects, and higher animals involve such fertility issues. Those insects that lay eggs, however, their eggs do not hatch, may involve problems in their genetic complements including lethality due to inversions. Lethality also arises in the hemizygous conditions involving recessive mutations.

Position effect:

It is best studied using Drosophila eye color as an example. Consider a case involving inversion resulting in the fusion of the genes responsible for the eye color with the genes present in the heterochromatic region. The expression of the red-eye allele, in such a case, results in the white-eyed phenotype. Sometimes, inversions bring euchromatin regions close to the heterochromatin regions. Such type of inversions leads to heterochromatinization of the euchromatin.

References:

[1] Emery's Elements of Medical Genetics, Peter D Turnpenny, Sian Ellard

[2] Chromosomal inversion, Wikipedia
[3] Chromosomal inversion, Biology-Online Dictionary

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