In this article we will discuss about:- 1. Meaning of Pericentric Inversion 2. Chromosome Morphology 3. Occurrence of Pericentric Inversion.
Meaning of Pericentric Inversion:
When the inverted segment is large, a loop is formed during synapsis and centromere is included in the loop. Crossing over in the loop causes the formation of deficiency-duplication chromatids. Chromosome pairing and crossing over positions within the inversion loop are diagrammatically represented in the Fig. 15.4.
Single crossing over at any position within the loop will produce two deficiency-duplication chromatids resulting in sterility of the gametes carrying them (Fig. 15.4). In such a type of crossing over, one of the non-crossover chromatids will be normal, while the other will be inverted; both of them will produce fertile gametes.
Double crossing over involving the same two chromatids (for instance, at positions I and II) will generate no change.
Dicentric chromatid is never formed due to crossing over in a pericentric inversion loop. Double crossing over involving 3 chromatids (3-strand double crossing over), e.g., at the positions I and III or I and IV, will also result into two deficiency-duplication chromatids, plus one normal and one inverted chromatids.
A 4-strand double crossing over, for example, at the positions I and V, will produce all the chromatids having deficiency-duplication; as a result, all the gametes will be nonfunctional. A summary of the results of different types of crossing over within a pericentric inversion loop is given in the Table 15.1.
Chromosome Morphology:
The morphology of inverted chromosome is changed in cases where the inverted segments on the two sides of the centromere are of different length. Pericentric inversions cause chromosome polymorphism as a result of which different karyotypes may originate.
The position of centromere may be changed from median to sub-median or sub-terminal positions (Fig. 15.5). Chromosome polymorphism arising due to pericentric inversion has been observed in some natural populations of grasshoppers (Morobascurra, Ailopusstrepens, Trimerotroplsgracilis).
Occurrence of Pericentric Inversion:
Pericentric inversions have been reported in several plant species, such as, maize, Viciafaba and in fungi. They have also been reported in animals including human. In many cases, they are not detected or they do not have a morphological effect. One of the reasons for this may be that the inverted fragment may be small and may remain unpaired, or if paired, crossing over may not occur.
In the absence of crossing over within the inverted segment, deficiency-duplication chromatids are not produced and consequently, there is no reduction in fertility. On the other hand in Drosophila, reduced gametic fertility may be the reason for the low frequency of observed pericentric inversions surviving in the natural populations.