29. During meiosis if one of the chromosome lack its homologous partner while all other chromosome are paired, such aberration is termed as
(1) Haploidy (2) Monosomic
(3) Trisomy (4) Nullisomy
Concept and reasoning
During meiosis in a normal diploid, each pair of homologous chromosomes forms a bivalent. If all but one chromosome have a partner, it means the organism has only one copy of that particular chromosome instead of the usual pair. That state is called monosomy (2n − 1).
Option‑wise explanation
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Haploidy
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Haploid cells have one complete set of chromosomes (n) instead of two (2n).
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In haploids, no chromosome has a homologous partner; all are single.
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The question says only one chromosome is unpaired while others are paired, so this is not haploidy.
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Monosomic – correct
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Monosomy is an aneuploidy where a specific chromosome is present as a single copy (2n − 1).
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In meiosis of a monosomic individual, that chromosome appears as a univalent (unpaired) while other chromosomes form normal bivalents—exactly as described.
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Trisomy
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Trisomy means there are three copies of a chromosome (2n + 1).
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Meiosis shows a trivalent or combinations of bivalent + univalent, not just a single unpaired chromosome with all others perfectly paired.
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Nullisomy
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Nullisomy is loss of both homologs of a chromosome pair (2n − 2).
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That chromosome is absent completely, so it cannot appear unpaired in meiosis.
Therefore, the aberration where one chromosome lacks its homologous partner while all other chromosomes pair normally is monosomy (option 2).
