Chromosomal inversion is also called by different names (genetic inversion, chromosome inversion or inversion mutation). The Inversions are chromosomal mutations that conclude a segment of a chromosome that is turned around 180o and is reinserted into the chromosome. It does not involve a loss of genetic information but simply rearranges the linear gene sequence. An inversion involves two breaks along the length of the chromosome before the reinsertion of the inverted segments.
Types of inversion
The inverted segment may be short or quite long and may or may not include the centromere. If the centromere is not the part of the rearranged chromosome segment, the inversion is said to be Paracentric, while if the centromere is a part of the inverted segment, the inversion is known as Pericentric.
Inversion Loop In Chromosomal Inversion
The organisms in which one inverted chromosome and one non-inverted homolog are present, are called Inversion Heterozygotes. Normal linear pairing between such chromosomes is not possible until they form an Inversion Loop as shown in Fig: 1.1.
In other cases, no loop can be found and the homolog is seen to synapse everywhere but along the length of the inversion, where they appear separated. If crossing over does not occur within the inversion segment of the inversion, where they appear separated.
If crossing over does not occur within the inverted segment of the inversion heterozygote, the homologs will segregate normally. When crossing over occurs within the inversion-loop abnormal chromatids are produced. In any meiotic tetrad, a single cross-over produces two parental chromatids and two recombinant chromatids.
In cases of a paracentric inversion, one recombinant is Dicentric, i.e., having two centromeres and one recombinant in Acentric, i.e., lacking a centromere. Both contain duplications and deletions of chromosome segments as well.
During anaphase, an acentric chromatid moves randomly to one pole or the other or maybe lost, while a dicentric chromatid is pulled in two directions. This polarized movement produces Dicentric Bridges which may be recognized cytologically.
A dicentric chromatid usually breaks, at some point so that part of the chromatid goes into the next gamete and another part into another gamete during the reduction division. Therefore, gametes in which gamete participates in fertilization, the zygote most often develops abnormally.
Chromosome Imbalance In Genetic Inversion
A similar chromosome imbalance is produced as a result of a cross over event between a chromatid bearing a pericentric inversion and its non-inverted homolog. Following meiotic divisions, each tetrad yields two parental chromatids containing the complete chromosome complement of genes.
The recombinant chromatids that are directly involved in exchange have duplications and deletions. However, no acentric or dicentric chromatids are produced. Gametes receiving these chromatids also produce variable embryos following their participation in fertilization.
Since fertilization involving these aberrant chromosomes do not produce viable offspring. It appears as if the inversion suppresses crossing over since cross over gametes is not recovered in the offspring. Actually, in inversion heterozygotes, the inversion has the effect of suppressing the recovery of cross over products when chromosome exchange occurs within the inverted region.
If crossing over always occurred within a paracentric inversion or pericentric inversion, 50 % of gametes would be ineffective. The viability of resultant zygote is, therefore, greatly diminished, Furthermore, up to one-half of the viable gametes have the inverted chromosomes, and the inversion will be perpetuated within the species. The cycle will be repeated continuously during meiosis in future generations.
The inversion involves the new positioning of genes relative to other genes. If the expression of a gene is altered as a result of its relocation, a change in phenotype may result. Such a change is called PositionEffect. In Drosophila females, heterozygous for sex-linked recessive mutation with an eye (w+/ w), the X chromosome bearing the wild-type allele (w) may be inverted, and the white locus moves to a point adjacent to centromeric heterochromatin.
If the inversion is not present the heterozygous female has wild-type red-eye since the white allele is recessive. Females with the X chromosome inversion have eyes that are mottled or variegated (with red and white patches). The relocation of the w+ allele next to a heterochromatin area seems to cause a loss of complete dominance over the w allele.
Other genes located on the X chromosome will also behave in the same manner if relocated. Reversion to wild type expression has sometimes been noted. When this has occurred, the cytological examination has shown that the inversion has been reversed to give the normal gene sequence.
Role of Inversion in Evolution
An inversion maintains a set of alleles at a series of adjacent loci provided they are contained within the inversion. Because the recovery of crossover products is suppressed in inversion heterozygotes, a particular gene sequence is preserved intact in the viable gametes.
If this gene order provides a survival advantage to an organism having it, the inversion is beneficial from an evolutionary point of view. For example, if the set of alleles ABcDeF is more adaptive than the sets AbCdeF or abcdEF, the favorable set will not be disrupted by the crossing over if it is maintained within a heterozygous inversion.
Theodosius Dobzhansky has shown the maintenance of different inversions on chromosome 3 of Drosophillapseudoobscura through many generations, have made this species highly adaptive. The adaptations help in selection during the process of evolution.
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