Q.54 If an egg cell of a diploid plant species has 10 chromosomes, the expected
number of chromosomes in a double trisomic somatic cell of this species
would be _______________ (Answer in integer).
In a diploid plant species where the egg cell (haploid gamete) contains 10 chromosomes, the somatic cells of the normal diploid plant have 20 chromosomes (2n = 20). A double trisomic somatic cell features two extra chromosomes, each from a different pair, resulting in a total chromosome count of 2n + 2 = 22.
Key Concepts
Egg cells form via meiosis, halving the chromosome number from diploid somatic cells, so n = 10 implies 2n = 20. Trisomy (2n + 1) adds one extra chromosome; double trisomy adds two extras from distinct pairs (2n + 1 + 1 = 2n + 2).
Step-by-Step Solution
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Haploid number (n) from egg cell: 10 chromosomes.
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Diploid somatic number (2n): 2 × 10 = 20.
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Single trisomic somatic cell: 20 + 1 = 21.
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Double trisomic somatic cell: 20 + 2 = 22.
No options are provided in the query, but common alternatives like 20 (normal diploid), 21 (single trisomic), or 24 (tetrasomic, 2n + 2 for same pair) are incorrect for double trisomy.
In diploid plant species where the egg cell has 10 chromosomes, understanding double trisomic somatic cell chromosome number is crucial for genetics and CSIR NET Life Sciences aspirants. This guide breaks down the calculation, clarifies aneuploidy types, and provides exam-ready insights.
Diploid Basics and Egg Cell Chromosomes
Plants typically have diploid somatic cells (2n chromosomes). Gametes like egg cells are haploid (n) after meiosis. If the egg cell has 10 chromosomes, then n = 10 and 2n = 20 in normal somatic cells. This sets the baseline for aneuploid conditions like trisomy.
What is Double Trisomy in Plants?
Trisomy occurs when one chromosome pair has three copies (2n + 1). Double trisomy (or double trisomic) adds extras from two different pairs: 2n + 1 + 1 = 2n + 2. Examples include plants like Datura or barley, where such mutants aid gene mapping. Unlike tetrasomy (extra pair of the same chromosome, also 2n + 2), double trisomy specifies distinct chromosomes.
Calculating Chromosomes in Double Trisomic Somatic Cell
For this species:
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Egg cell: n = 10 → somatic diploid: 2n = 20.
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Double trisomic somatic cell: 20 + 2 = 22 chromosomes.
| Condition | Formula | Chromosomes (n=10) |
|---|---|---|
| Normal diploid | 2n | 20 |
| Single trisomic | 2n + 1 | 21 |
| Double trisomic | 2n + 2 | 22 |
| Tetrasomic | 2n + 2 (same pair) | 22 |
| Nullisomic | 2n – 2 | 18 |
Relevance for CSIR NET and Plant Genetics
Double trisomics are studied in cytogenetics for chromosome behavior and breeding. They show reduced fertility due to meiotic irregularities but help locate genes (e.g., in tomato trisomics). For exams, distinguish from polyploidy (whole-set changes).
Master double trisomic somatic cell chromosome number for competitive exams—practice with n=10 yields 22!
