Describe inversions and translocations in the structure of chromosomes

INVERSIONS:

The inversion is a type of chromosomal aberration in which a segment chromosome is turned around 180° and inserted into the chromosome. An inversion 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 prior to the reinsertion of the inverted segment.

In those cases where the centromere is not the part of the rearranged chromosome segment, the inversion is said to be paracentric on other other hadn if the centrosome is a part o the inverted segment, the inversion is known as Pericentric.

The organism with one inverted chromosome and one non-inverted homologue present, are called inversion heterozygotes.

Pairing between such chromosome is not possible until they form an inversion loop. If crossing over does not occur within the inverted segment of the inversion heterozygote, the homologues will segregate normally. When crossing over occurs within the inversion loop, abnormal chromatids are produced. A single cross over produces two parental chromatids and two recombinant chromotids. In case of a paracentric inversion one recombinant is Dicentric i.e. having two centrosome and one recombinant is a centric i.e. lacking a centromere. Both contain duplications and deletions of chromosome segments as well. During avaphase, an acentric chromatid mones roudomly to one pole or the other or may be lost, while a decentric chromatid is pulled in two directions. 

This polarized movements produces Dicentric Briodges. A deventric chewmated break at some point so that part of the chromatid goes into newxt gamete and other part into another gamete during the reduction division. In this way gametes which contain other recombinant chromatid are deficient in genetic material when such a gamete participates in fertilization the zygote most often develops abnormally.

During pericentric inversion each tetrad yield two parental chromatids containing complete set of genes and the recompinant chromatids have duplications and deletions as they are diresctly involved inc crossing over. No acentric or dicentric chromatids are produced. Gametes receiving these chromatids also produce inviable embryos.

The inversion results in 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 phemotype may result. Such a change is called position effect. In Drosophhilla females, heterozygous for sex linked recesive mutation with eye (W⁺/W/, X chromosome bearing the wild tupe allele (w) amy be inverted and the while locus moves to a point adjacent to centro metric heterochromatin. If the inversion is not present the heterozygous female ahs wild type red eye because the white allele is recessive. Females with X chromosome inversion have eyes that are mottled or variegated having red and white patches. Relation of W⁺ allele next to a heterochromatin area seems to cause a loss of complete dominance over the recessive allele. Other genes located on X chromosome will also behave in the same manner of shifted to some other place.

Genetic consequences of Inversion:

The process of inversion maintains a set of alleles at a series of adjacent loco provided they are contained within inversion. Because the recovery of cross over 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 organism having it, the inversion is beneficial from evolutionary point of view. For example if the set of alleles AB, DeF is more adaptive than the sets AbcDEF or abcdEF, the favourable set will not be disturbed by the crossing over if it is maintained within a heterozygous inversion.

TRANSLOCATIONS:

The transfer of a section of one chromosome to a non homolofue is called tranlocation. In Drosophilla, translocation was first recognised gentetically by the unusual behaviour of second chromosome gene known as pale, which had the phemotypic effect of diluting certain eye colours. Although pale was lethal in homozygous conditions. Bridges found that its lethality and phenotypic effect could be suppressed by the presence of another gene present on third chromosome which is also lethal when homozygous. The lethality of the latter, in turn, was suppressed by the presence of the former, linkave analysis soon sowed that the pale effect was caused by deficiency for a small section of genes on the tip of second chromosome which had linked to third chromosome between genes ebony and rough. In other words deficiency and translocation has transferred a gene from chromosome 2^nd to 3^rd.

At present a variety of translocation are known among which following are most common.

(1)        Simple translocation: They are produced by single break in chromosome and transfer of a broken piece of this chromosome directly into the end of another.

(2)        Shifts or Intercalary Translocation: They are more common and are produced by involving three breaks so that two break section of one chromosome is inserted within the break produced in non homologous chromosome.

(3)        Reciprocal Translocation: These are interchanges which occur when single breaks into two non homologous chromosomes produce an exchange of chromosome sections between them.

GENETICS CONSEQUENCES OF TRANSLOCATIONS:

(1)        Process of translocation provides of one the proof that genes are present on chromosomes.

(2)        Translocation has also helped us to understand the position effect i.e. when a chromosome rearrange ment involves no change in the amount of genetic material but only in the order of genes, the term position effect is used to describe any associated phenotypic alteration.

(3)        Translocation sometimes is the cause of sterility.