Q1. [3] § 8.2.2 Rules for the Inheritance of Traits – Mendel's Contributions
The gene combination of purple flowered pea plants is denoted as (WW) and that of white flowered pea plants as (ww), when these two plants are crossed F₁ generation is obtained.
(a) List two observations made by Mendel in F₁ generation plants.
(b) Give the (i) percentage of white flowered plants and (ii) ratio of the gene combinations WW, Ww and ww in F₂ generation.
(c) Write one difference between dominant and recessive trait.
Previously asked in CBSE board exam
2025 31/6/1 Q30
Generated by claude-sonnet-4-6 · 2026-06-15 06:53 · grounding rag
Model Answer
(a) Two observations in F₁ generation:
- All plants had purple flowers — only one parental trait was expressed.
- There were no intermediate forms (no "light purple" flowers).
(b)
(i) 25% white-flowered plants in F₂ generation.
(ii) Ratio of WW : Ww : ww = 1 : 2 : 1
(c)
| Dominant Trait | Recessive Trait |
|---|---|
| Expressed even when a single copy of the gene is present (e.g., WW or Ww) | Expressed only when both copies are identical recessive genes (ww) |
Source: Heredity, Section 8.2.2
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Explanation
- (a) Examiners expect: (1) only one trait expressed / no blending, and (2) all F₁ plants look alike (all purple). Both are directly from Mendel's observations.
- (b) The cross Ww × Ww gives 1WW : 2Ww : 1ww. Only ww = white = 1/4 = 25%. State both the percentage and the ratio clearly.
- (c) The key distinction: dominant needs only one copy; recessive needs two copies (homozygous). A one-line tabular format is clean and scores full marks for a 1-mark difference question.
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