10. Inversions are considered as cross-over suppressors because:
(1) Homozygous inversions are lethal and thus they do not appear in next generation.
(2) Inversion heterozygotes, i.e., one copy having normal chromosome and its homologue having inversion, does not allow crossing over to occur as they cannot pair at all.
(3) Due to inversion present. Four chromosomes take part in the pairing and crossing over events and make the structure difficult for separation and gamete formation.
(4) The pairing and crossing overs do occur in inversion heterozygotes but the gametes having cross over products are lethal.

The correct answer is (4) The pairing and crossing overs do occur in inversion heterozygotes but the gametes having cross over products are lethal.

Core concept

In a chromosome with an inversion paired with its normal homolog (inversion heterozygote):

• Homologs form an inversion loop to align genes.

• If a crossing over occurs inside this loop, the resulting recombinant chromatids are structurally unbalanced:

  • For paracentric inversions: acentric and dicentric chromatids form.

  • For pericentric inversions: chromatids carry segmental deletions and duplications.

• Such gametes are usually non‑viable or give lethal zygotes.

• Therefore, although crossing over happens, only non‑recombinant (parental) chromatids are recovered, so inversions appear to suppress crossing over.

This is exactly what statement (4) says.

Option‑wise explanation

1. Homozygous inversions are lethal and thus they do not appear in next generation.

   • Most inversions in homozygous condition are actually viable because gene content is unchanged.

   • Crossover suppression is a property of heterozygotes, not homozygotes, so this is incorrect.

2. Inversion heterozygotes … do not allow crossing over to occur as they cannot pair at all.

   • Inversion heterozygotes can pair by forming an inversion loop, and crossing over does occur inside that loop.

   • The term “crossover suppressor” does not mean crossovers never happen; it means recombinant products are not recovered.

3. Due to inversion present, four chromosomes take part in the pairing and crossing over events…

   • That description fits certain reciprocal translocations (quadrivalent formation), not simple inversions, which involve only one homologous pair (two chromosomes).

4. The pairing and crossing overs do occur in inversion heterozygotes but the gametes having cross over products are lethal. – correct

   • Accurately states that pairing and crossover occur, but recombinant gametes are defective, so only non‑crossover gametes survive, giving the observed “crossover suppression.”

Thus, the mechanistic reason inversions are considered crossover suppressors is best captured by option (4).