9. Inversion is termed as crossover suppressor because
(1) Single crossing over within an inversion loop, leads to defective (deleted and duplicated) crossoverchromosomes and mortality of zygotes carrying them.
(2) No crossing over in the inversion loop
(3) Crossing over lead to formation of all acentric chromosomes
(4) Segregation of chromosomes in not normal
The correct answer is (1) Single crossing over within an inversion loop leads to defective (deleted and duplicated) crossover chromosomes and mortality of zygotes carrying them.
Concept
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In an inversion heterozygote, homologous chromosomes pair by forming an inversion loop.
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If a single crossover occurs within this loop:
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For a paracentric inversion: acentric and dicentric chromatids form and are usually lost.
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For a pericentric inversion: each recombinant has one centromere but carries segmental deletions and duplications.
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These recombinant gametes generally produce non‑viable zygotes, so only gametes without crossovers (parental chromatids) contribute to offspring.
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Thus, crossing over technically occurs, but recombinant products are eliminated, giving an apparent suppression of crossovers.
Option‑wise explanation
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Single crossing over within an inversion loop, leads to defective (deleted and duplicated) crossover chromosomes and mortality of zygotes carrying them – correct
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Captures the real reason for “crossover suppression”: crossovers do happen, but the resulting recombinant chromatids are structurally unbalanced (deletions/duplications or acentric/dicentric) and not recovered in viable progeny.
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No crossing over in the inversion loop
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Not accurate. Crossing over can occur; it is the fate of recombinant chromatids (they are inviable) that leads to suppression at the genetic level.
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Crossing over leads to formation of all acentric chromosomes
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For paracentric inversions, one recombinant is acentric and another is dicentric, not “all acentric”; and for pericentric inversions the recombinants are not acentric at all. So this statement is incorrect and incomplete.
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Segregation of chromosomes is not normal
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Segregation problems can occur, but this is a consequence of abnormal recombinants, not the core reason inversions are termed crossover suppressors. The key feature is the production and loss of defective recombinant chromatids, as described in option (1).
Therefore, inversion is called a crossover suppressor because single crossovers within the inversion loop produce deleted/duplicated recombinant chromosomes that lead to inviable gametes or zygotes, so only non‑crossover products are recovered—matching option (1).
