(A) 12
(B) 13
(C) 15
(D) 16
Nullisomy represents loss of both homologous chromosomes of one pair, a severe aneuploidy (2n-2) often lethal in diploids but viable in polyploids like wheat. For 2n=14 (n=7 chromosomes), nullisomic loses one entire chromosome pair, resulting in 12 chromosomes.
The correct answer is (A) 12, as nullisomic = 2n – 2 = 14 – 2 = 12 chromosomes.
Nullisomy Definition: Complete Loss of One Chromosome Pair
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Diploid (2n=14): 7 homologous pairs (14 total chromosomes)
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Nullisomic: One pair missing → 6 pairs remain (12 chromosomes)
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Notation: 2n-2 (loses both homologs of chromosome #X)
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Meiosis: 6 bivalents (no univalent or trivalent formation)
Aneuploidy Chromosome Number Comparison
| Condition | Chromosome Change | 2n=14 → Result | Example |
|---|---|---|---|
| Normal | – | 14 | Wild type |
| Monosomic | -1 chromosome | 13 | 2n-1 |
| Nullisomic | -2 chromosomes (1 pair) | 12 | 2n-2 |
| Trisomic | +1 chromosome | 15 | 2n+1 |
| Tetrasomic | +2 chromosomes | 16 | 2n+2 |
Explanation of All Options
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(A) 12
Correct. Nullisomy = loss of both homologs of one chromosome pair. 14 – 2 = 12 chromosomes remain. -
(B) 13
Wrong. Monosomic (2n-1) loses only one chromosome from a pair, leaving 13 total. -
(C) 15
Incorrect. Trisomic (2n+1) gains extra chromosome, creating 15 total. -
(D) 16
No. Tetrasomic (2n+2) or double trisomic results in 16 chromosomes.
Visual Chromosome Pair Loss
Normal (2n=14): [1a,1b] [2a,2b] [3a,3b] [4a,4b] [5a,5b] [6a,6b] [7a,7b]
Nullisomic (2n-2=12): --- [2a,2b] [3a,3b] [4a,4b] [5a,5b] [6a,6b] [7a,7b]
↑ Pair #1 missing entirely
Biotech relevance: Nullisomics used in wheat breeding to identify homoeologous chromosome groups. Nulli-tetrasomic combinations (2n-2+4=16) compensate gene loss. Memory trick: “Nulli = NONE of that chromosome pair → subtract 2.” Common confusion: monosomic (13) vs nullisomic (12).
