- Inversion involves a rotation of a part of a chromosome or a set of genes by 180° on its own axis.
- Breakage and reunion is essential for inversion to occur and the net result is neither a gain or loss in the genetic material but simply a rearrangement of the gene sequence.
- Inversion that include the centromere are known as pericentric (around the centromere), whereas those which do not involve the centromere are known as paracentric ones.
- If the normal sequence of genes in a chromosome is ABC.DEFG, the sequence in paracentric and pericentric inversions will be ABC.DGFE and AED.CBFG, respectively.
- In organisms which are homozygous for inversions, zygotene and pachytene is normal because of similarity of abnormalities in the homologous chromosomes.
- But heterozygous inversions result into looped configurations during pachytene.
- If the heterozygous inversion is small enough, the opposing inverted regions fail to pair and crossing-over is suppressed in this part of the chromosome.
- A crossing-over in the inverted region of a heterozygous paracentric inversion results into a chromosome with two centromeres and an acentric fragment.
- As the two centromere of the dicentric chromosome move towards two opposite poles, a chromatid bridge is formed at anaphase I.
- The acentric fragment is not attached to the spindle, lies free in the cytoplasm and is not disjuncted properly.
- Thus meiotic separations are usually abnormal.
- In case meiotic separation is normal, four types of gametes are formed – one with a normal gene sequence, second with an inverted gene sequence, third with a dicentric chromosome and duplication of some genes, and fourth with an acentric chromosome and deletion of some genes.
- The latter two types of gametes are usually not viable, with the result that heterozygotes for paracentric inversions are highly sterile and only parent-like progeny are produced.
- In other words, crossing-over is apparently suppressed due to paracentric inversion.
- Dicentric bridge formed in maize female tissue as a result of such crossing-over results into a bridge-breakage-fusion cycle.
- Pollen grains show a high degree of sterility.
- Multiple cross-overs reduced the suppression effect, especially with respect to those genes between which two (or even number of) crossovers occur, because their result is the same as no cross-over (only if the same two strands are involved in even number of exchanges).
- Crossing-over in a heterozygous pericentric inversion does not result into a chromatid bridge, but results into deletions and duplications in the gametes.
- Therefore, pericentric inversions also apparently suppress crossing-over.
- Although heterozygotes for pericentric inversions produce a significant proportion of inviable gametes and unbalanced zygotes as a rule,
- Drosophila is an exception because in the females there is reduced paring and consequent reduced crossing-over in the inverted region between the homologous chromosomes.
- Pericentric inversions involving unequal arms result into drastic changes in the morphology of chromosomes.
- For example, metacentric (V-shaped) chromosomes can be transformed into acentric (rod-shaped) ones or vice-versa.
- Fertility of inversion of homozygote and sterility of inversion heterozygote leads to the establishment of two groups which are mutually fertile but do not breed well with the rest of species.
- Thus, two varieties are established which evolve in different directions and later become reproductively isolated species.
- There is plenty of cytological evidence to prove that such evolutionary mechanisms have and are operating in Drosophila and a number of other organisms.
- Inversion has been useful in establishing and maintaining a heterozygous condition, because in inversion heterozygotes crossing-over is suppressed and only parental progeny are produced.
- Recessive lethal can be added advantage because heterozygotes for them will be viable but homozygote nonviable.
- Quite a few strains of Drosophila are routinely maintained with the help of lethal genes and inversion heterozygosity.
MCQs on Inversion (with PYQs)
Basic Concepts
- Inversion in chromosomes involves:
a) Deletion of a gene
b) Duplication of a gene
c) Rotation of a chromosome segment by 180°
d) Addition of genes
Answer: c) Rotation of a chromosome segment by 180° - Which process is essential for inversion to occur?
a) Transcription & Translation
b) Breakage and reunion
c) DNA replication
d) Mutation repair
Answer: b) Breakage and reunion - In inversion, the net result is:
a) Loss of genetic material
b) Gain of genetic material
c) Rearrangement of gene sequence
d) Polyploidy
Answer: c) Rearrangement of gene sequence - Inversion involving the centromere is called:
a) Paracentric inversion
b) Pericentric inversion
c) Isochromosome
d) Acentric inversion
Answer: b) Pericentric inversion - Inversion that does not involve the centromere is called:
a) Paracentric inversion
b) Pericentric inversion
c) Isochromosome
d) Telomeric inversion
Answer: a) Paracentric inversion
Gene Sequence Examples
- Normal gene sequence: ABC.DEFG. After paracentric inversion, the sequence is:
a) AED.CBFG
b) ABC.DGFE
c) AB.DEFGC
d) ACB.DEFG
Answer: b) ABC.DGFE - The same sequence after pericentric inversion becomes:
a) AED.CBFG
b) ABC.DGFE
c) ACB.DEFG
d) ABF.CDEG
Answer: a) AED.CBFG
Cytology & Pairing
- Homozygotes for inversion show:
a) Normal zygotene & pachytene
b) Abnormal pachytene loops
c) No crossing-over suppression
d) No pairing
Answer: a) Normal zygotene & pachytene - Heterozygotes for inversion show during pachytene:
a) Straight pairing
b) Looped configuration
c) Multiple fragments
d) No pairing
Answer: b) Looped configuration - Small heterozygous inversion regions lead to:
a) High crossing-over
b) Suppression of crossing-over
c) Gene duplication
d) Gene deletion
Answer: b) Suppression of crossing-over
Paracentric Inversion Effects
- Crossing-over in a paracentric inversion leads to:
a) Dicentric + acentric fragments
b) Only balanced gametes
c) Gene amplification
d) Polyploidy
Answer: a) Dicentric + acentric fragments - The dicentric chromosome results in:
a) Chromatid bridge at anaphase I
b) Gene duplication only
c) Suppression of pairing
d) Free fragments
Answer: a) Chromatid bridge at anaphase I - The acentric fragment:
a) Attaches to spindle fibers
b) Lies free in cytoplasm
c) Always transmitted
d) Becomes centric again
Answer: b) Lies free in cytoplasm - Four types of gametes formed in paracentric inversion are:
a) All viable
b) Two viable, two non-viable
c) All sterile
d) All fertile
Answer: b) Two viable, two non-viable - Why are heterozygotes for paracentric inversion highly sterile?
a) Excessive duplication
b) Formation of dicentric & acentric chromosomes
c) Polyploidy
d) Lack of recombination
Answer: b) Formation of dicentric & acentric chromosomes - Suppression of crossing-over due to inversion is called:
a) Pseudo-suppression
b) Apparent suppression
c) Genetic drift
d) Crossing-over interference
Answer: b) Apparent suppression - In maize female tissue, crossing-over in paracentric inversion produces:
a) Ring chromosomes
b) Bridge-breakage-fusion cycle
c) Robertsonian translocation
d) Isochromosomes
Answer: b) Bridge-breakage-fusion cycle - Pollen grains with paracentric inversion show:
a) Complete fertility
b) High sterility
c) Polyploid gametes
d) Super-fertility
Answer: b) High sterility - Multiple crossovers in inversion heterozygotes:
a) Increase sterility
b) Reduce suppression effect
c) Stop gamete formation
d) Cause duplication
Answer: b) Reduce suppression effect
Pericentric Inversion Effects
- Crossing-over in pericentric inversion leads to:
a) Dicentric bridge formation
b) Deletions & duplications
c) Balanced gametes only
d) Isochromosomes
Answer: b) Deletions & duplications - In pericentric inversion heterozygotes, fertility is reduced due to:
a) Polyploidy
b) Inviable gametes & unbalanced zygotes
c) Telomere shortening
d) Lack of meiosis
Answer: b) Inviable gametes & unbalanced zygotes - Which organism shows exception to this general rule?
a) Maize
b) Drosophila
c) Pea
d) Wheat
Answer: b) Drosophila - Reason for Drosophila’s exception in pericentric inversion:
a) Extra centromeres
b) Reduced pairing & crossing-over in females
c) Polyteny
d) Gene amplification
Answer: b) Reduced pairing & crossing-over in females - Pericentric inversions involving unequal arms can change chromosome morphology from:
a) Submetacentric ↔ Isochromosome
b) Metacentric ↔ Telocentric
c) Polyploid ↔ Diploid
d) Rod ↔ Ring chromosome
Answer: b) Metacentric ↔ Telocentric
Evolutionary Importance
- Inversion homozygotes are usually:
a) Sterile
b) Fertile
c) Lethal
d) Non-viable
Answer: b) Fertile - Inversion heterozygotes are often:
a) Fertile
b) Sterile
c) Polyploid
d) Aneuploid
Answer: b) Sterile - This fertility difference leads to:
a) Genetic drift only
b) Establishment of two mutually fertile groups
c) No evolution
d) Transposon formation
Answer: b) Establishment of two mutually fertile groups - Such groups can evolve into:
a) Inbreeding species
b) New reproductively isolated species
c) Mutant strains only
d) Haploid populations
Answer: b) New reproductively isolated species - Cytological evidence of inversion’s role in evolution is strongest in:
a) Wheat
b) Rice
c) Drosophila
d) Human
Answer: c) Drosophila
Utility in Genetics
- Inversions are useful in maintaining:
a) Polyploidy
b) Heterozygosity
c) Homozygosity
d) Apomixis
Answer: b) Heterozygosity - Crossing-over suppression in inversion heterozygotes results in:
a) Recombinant progeny
b) Only parental progeny
c) Sterile progeny only
d) Polyploids
Answer: b) Only parental progeny - Inversions combined with recessive lethal genes are useful because:
a) Homozygotes viable, heterozygotes sterile
b) Heterozygotes viable, homozygotes non-viable
c) Both viable
d) Both sterile
Answer: b) Heterozygotes viable, homozygotes non-viable - Drosophila strains are often maintained using:
a) Transposons
b) Polyploidy
c) Inversion heterozygosity + lethal genes
d) Isochromosomes
Answer: c) Inversion heterozygosity + lethal genes
PYQs (Past Year Questions)
(Integrated into the set – exam years given where known)
- Paracentric inversion differs from pericentric inversion in that it:
a) Involves centromere
b) Excludes centromere ✅ (NEET PYQ)
c) Produces only viable gametes
d) Causes polyploidy
Answer: b) Excludes centromere - A dicentric bridge and acentric fragment are characteristic of:
a) Deletion
b) Duplication
c) Paracentric inversion ✅ (CSIR NET PYQ)
d) Pericentric inversion
Answer: c) Paracentric inversion - A crossing-over in pericentric inversion produces:
a) Chromatid bridge
b) Deletions and duplications ✅ (BSc Univ Exam PYQ)
c) Ring chromosome
d) Isochromosome
Answer: b) Deletions and duplications - Suppression of crossing-over in inversion heterozygotes leads to:
a) Only parental type progeny ✅ (CUET PYQ)
b) New recombinants
c) Hybrid sterility
d) Polyploid gametes
Answer: a) Only parental type progeny - The bridge-breakage-fusion cycle in maize is due to:
a) Duplication
b) Translocation
c) Paracentric inversion ✅ (ICAR JRF PYQ)
d) Isochromosome
Answer: c) Paracentric inversion - Fertility in inversion homozygotes and sterility in heterozygotes can lead to:
a) Genetic recombination
b) Speciation ✅ (UPSC PYQ)
c) Gene loss
d) Apomixis
Answer: b) Speciation - Which of the following is an example of inversion leading to evolutionary isolation?
a) Drosophila ✅ (CSIR-NET PYQ)
b) Wheat
c) Maize
d) Yeast
Answer: a) Drosophila
Conceptual / Higher-order
- Which type of inversion alters chromosome morphology drastically?
Answer: Pericentric inversion involving unequal arms - Which type of gametes formed in paracentric inversion are usually viable?
Answer: Gametes with normal sequence & inverted sequence - Why are dicentric chromosomes problematic during meiosis?
Answer: They form bridges during anaphase I - Which type of inversion is more harmful for fertility?
Answer: Paracentric inversion - Why does crossing-over suppression occur in inversion heterozygotes?
Answer: Inverted regions fail to align properly - Which organism shows reduced pairing in inversion heterozygotes, lowering crossing-over?
Answer: Drosophila females - What happens to an acentric fragment during anaphase?
Answer: It is lost in cytoplasm, not segregated - In maize, what cytological abnormality confirms paracentric inversion?
Answer: Dicentric bridge formation - Why are multiple crossovers sometimes not harmful in inversion heterozygotes?
Answer: Even number of crossovers between same strands cancel the effect - Geneticists maintain Drosophila strains using inversions because:
Answer: Inversions suppress crossing-over and maintain heterozygosity