Drosophila Polytene Chromosomes
Inversions result when a segment of a chromosome is excised, inverted 180o, and reintegrated into the same chromosome. Two specific types of inversions are recognized, and they are classified as to whether or not the centromere is a portion of the inverted segment.
Paracentric inversion - an inversion of a segment of the chromosome that does not involve the centromeric region
Pericentric inversion - an inversion of a segment of the chromosome that involves the centromeric region
In humans, it is difficult to detect inversions. If a large inversion occurs, then a new banding pattern will be seen in the regions that under went the inversion. One manner in which inversions can be detected is by a change in the location of the centromere. In the following example a pericentric inversion generates a metacentric chromosome from an acrocentric chromosome.
Inversions are not easily detected in homozygotes of other species, unless as with humans, distinct chromosomal banding patterns have been established for normal members of that species. Heterozygotes derived from a cross of a normal individual and an individual with an inversion in one chromosome will have specific patterns that occur because of pairing problems. In Drosophila, polytene chromosome will contain loops, evidence that one of the synapsed chromosomes contains an inversion.
The appearance of meiotic chromosomes also will be affected if one of the chromosomes has an inversion. Loops appear and define the region in which the inversion has occurred. Furthermore, if recombination occurs in a paracentric inversion region, then one-half of the gametes will be normal and the other half will contain deleted chromosomes. If the recombination involves a pericentric inversion then one-half of the gametes will be normal with respect to the initial chromosomal arrangements and the other gametes will contain chromosomes that contain both additions and deletions. These abnormalities can generate unbalanced gametes and in general, individuals with pericentric inversions are reproductively inferior.
In general, inversions do not change the phenotype of the individual unless an excision site of the inversion is within the regulatory or structural region of a gene. The primary change that is seen with inversions is a change in linkage relationships. A survey of 50 known paracentric inversions in humans revealed that:
Copyright © 1997. Phillip McClean