Q49. Arabidopsis and rice have diploid chromosome numbers of 10 and 24, respectively. Assuming no crossing over taking place, genetic variation among F2 individuals in a genetic cross is likely to be
Genetic variation in F2 individuals of Arabidopsis and rice without crossing over depends on independent assortment during meiosis.
Question Breakdown
Arabidopsis thaliana (2n=10, so n=5 chromosomes) and rice (Oryza sativa, 2n=24, so n=12 chromosomes) are diploid plants. In a genetic cross (e.g., AaBb × AaBb for two heterozygous loci), F1 progeny are uniform heterozygotes. F2 arises from F1 selfing or intercrossing, where meiosis produces gametes. Without crossing over, genes on the same chromosome stay linked, but different chromosomes assort independently per Mendel’s second law. Genetic variation in F2 thus comes solely from segregation and independent assortment—not linkage breaking.
Option Analysis
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(A) Same in both species but not zero: Incorrect. Variation scales with chromosome number; more chromosomes mean more independent units, generating more unique gamete combinations (2^n total possible gametes). Arabidopsis yields 2^5=32 gamete types; rice yields 2^12=4096. Punnett squares for F2 yield different genotypic ratios and diversity levels.
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(B) More in Arabidopsis: Incorrect. Fewer chromosomes (5 pairs) limit assortment events to 5, producing fewer gamete genotypes than rice’s 12 pairs. Lower chromosome count reduces combinatorial possibilities exponentially.
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(C) More in rice: Correct. Rice’s 12 chromosome pairs enable 4096 gamete types via independent assortment (2^12), versus Arabidopsis’s 32 (2^5). F2 individuals reflect random gamete unions, so rice F2 shows vastly higher genotypic variation (e.g., more heterozygous combinations). No crossing over preserves linkage but doesn’t eliminate inter-chromosomal shuffling.
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(D) Zero in both the species: Incorrect. Even without crossing over, independent assortment of chromosomes creates variation unless fully homozygous (not the case in heterozygous F1 crosses).
Introduction: Unlocking Genetic Variation in F2 Without Crossing Over
In plant genetics, particularly for competitive exams like GATE Life Sciences, understanding genetic variation F2 Arabidopsis rice no crossing over is crucial. Arabidopsis (diploid chromosome number 10) and rice (24) differ in genome structure, impacting F2 diversity in crosses absent crossing over. This article solves Q49 step-by-step, explains meiosis-driven variation, and aids biology students in Jaipur or worldwide prepping for exams.
Core Concept: Independent Assortment Drives F2 Variation
Meiosis in F1 heterozygotes shuffles chromosomes randomly at metaphase I. With no crossing over:
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Gametes form via segregation (alleles separate) and independent assortment (chromosomes align variably).
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Possible gametes = 2^n (n=haploid number).
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Arabidopsis: n=5 → 32 gametes → limited F2 phenotypes/genotypes.
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Rice: n=12 → 4096 gametes → exponential variation increase.
This holds for multi-locus traits across chromosomes; linked genes within chromosomes reduce but don’t nullify inter-chromosomal diversity.
| Species | Diploid (2n) | Haploid (n) | Gamete Types (2^n) | F2 Variation Level |
|---|---|---|---|---|
| Arabidopsis | 10 | 5 | 32 | Lower |
| Rice | 24 | 12 | 4096 | Higher |
Why Rice Shows More Genetic Variation in F2
Rice’s higher chromosome count amplifies assortment possibilities. For example, a dihybrid cross (two unlinked loci) yields 9:3:3:1 in both, but scaling to genome-wide loci, rice’s 12 chromosomes create vastly more recombinant-like combinations via assortment alone. Studies confirm recombination hotspots exist, but absent crossing over, assortment dominates variation. Arabidopsis’s compact genome limits this.
Exam Tips for GATE Life Sciences
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Focus on haploid number for assortment calculations.
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Remember: No crossing over ≠ no variation (unless n=1 or homozygous).
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Practice: Maize (2n=20, n=10) vs. rice shows similar rice advantage.
This detailed breakdown ensures mastery of genetic variation F2 Arabidopsis rice no crossing over for your prep.
