Q.64 In garden pea, dwarf plants with terminal flowers are recessive to tall plants with axial
flowers. A true–breeding tall plant with axial flowers was crossed with a true–breeding
dwarf plant with terminal flowers. The resulting F1 plants were testcrossed, and the
following progeny were obtained:
Tall plants with axial flowers = 320
Dwarf plants with terminal flowers = 318
Tall plants with terminal flowers = 79
Dwarf plants with axial flowers = 83
The map distance between the genes for plant height and flower position is______cM.
20
Genetic Cross Setup
True-breeding tall axial (T A / T A) crossed with dwarf terminal (t a / t a) yields F1 all tall axial (T A / t a), indicating genes are linked in cis configuration. Testcross of F1 with dwarf terminal (t a / t a) produces progeny reflecting parental and recombinant gametes from F1 meiosis.
Progeny Classification
Parental types: tall axial (320) and dwarf terminal (318), totaling 638, from non-recombinant gametes T A and t a. Recombinant types: tall terminal (79) and dwarf axial (83), totaling 162, from crossing over between loci.
Map Distance Calculation
Total progeny equals 800. Recombination frequency is recombinants divided by total: 162800=0.2025, or 20.25%. Map distance is 20 cM after rounding to nearest whole number for standard reporting.
Garden pea map distance calculation from testcross progeny is key for CSIR NET genetics prep. This garden pea linkage problem analyzes tall plants with axial flowers versus dwarf with terminal, using true-breeding parents and F1 testcross data.
Linkage and Testcross Basics
In garden peas, tall (T) dominates dwarf (t); axial flowers (A) dominate terminal (a). True-breeding tall axial (T A / T A) × dwarf terminal (t a / t a) gives F1 (T A / t a), all tall axial since linked. Testcross to t a / t a reveals linkage via progeny ratios deviating from 1:1:1:1 independent assortment.
Progeny Data Breakdown
Observed: tall axial 320, dwarf terminal 318 (parentals); tall terminal 79, dwarf axial 83 (recombinants) [query]. Total progeny: 800. Parental classes ~80%, recombinants ~20% confirm linkage.
| Phenotype | Count | Type |
|---|---|---|
| Tall axial | 320 | Parental |
| Dwarf terminal | 318 | Parental |
| Tall terminal | 79 | Recombinant |
| Dwarf axial | 83 | Recombinant |
Step-by-Step Solution
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Identify parentals: Most frequent, matching original combinations.
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Sum recombinants: 79 + 83 = 162.
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Recombination frequency: 162800×100=20.25%.
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Map distance: 20 cM (1% RF = 1 cM).
No options provided; direct fill-in yields 20. For CSIR NET, recognize cis linkage from data.
Exam Relevance
This mirrors CSIR NET two-point cross questions testing RF formula. Practice identifies coupling phase and avoids errors like including all progeny as recombinants.
