6. Chromosomal inversions are balanced rearrangements and thus do not change the overall amount of genetic material. While inversions can exist in homozygous condition, some only exist as heterozygotes. In the latter condition, the breakpoint disrupts:
(1) pairing of the homologous chromosomes
(2) a non-coding region of the genome
(3) a coding region of the genome
(4) a gene with an essential function
Concept
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Inversions usually do not change the total amount of DNA; they simply flip a segment.
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If both chromosomes carry the same inversion (homozygote), pairing and meiosis are generally normal and the organism is viable.
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However, some inversions are never seen in homozygous form because homozygotes are lethal: this happens when a breakpoint cuts directly through a gene that is essential for survival.
In a heterozygote, the intact allele on the normal chromosome can still supply the essential function, so carriers are viable. In a homozygote, both copies of that gene are disrupted, causing lethality; hence the inversion only exists as a heterozygote.
Option-wise explanation
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Pairing of the homologous chromosomes
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Inversion heterozygotes can still pair via an inversion loop; pairing problems mainly affect recombination and fertility, not the viability of homozygotes.
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This does not explain why the inversion “cannot” be homozygous.
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A non‑coding region of the genome
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Breaks in non‑coding DNA often have little or no direct effect on viability; an inversion with such breakpoints can easily be homozygous and still be harmless.
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A coding region of the genome
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Not every coding region is essential. If the breakpoint hits a non‑essential gene, homozygotes may still be viable.
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This is too general to guarantee lethality in homozygotes.
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A gene with an essential function – correct
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If the breakpoint disrupts an essential gene, heterozygotes survive thanks to the intact copy on the normal chromosome, but homozygotes would have both copies broken and die.
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Therefore the inversion persists only in the heterozygous state, exactly as described in the question.
