Q1. [4] § 8.2.2 Rules for the Inheritance of Traits – Mendel's Contributions
Mendel blended his knowledge of Science and mathematics to keep the count of the individuals exhibiting a particular trait in each generation. He observed a number of contrasting visible characters controlled in pea plants in a field. He conducted many experiments to arrive at the laws of inheritance.
Read the case and answer the following questions:
- (c) If 1600 plants were obtained in F2 progeny, write the number of plants having traits :
(i) Tall with round seeds
(ii) Short with wrinkled seeds
Write the conclusion of the above experiment.
Previously asked in CBSE board exam
2022 31/2/1 Q14 (OR-2)
Generated by claude-sonnet-4-6 · 2026-06-15 06:54 · grounding stimulus
Model Answer
(c)
In F₂ progeny from a dihybrid cross, the phenotypic ratio is 9 : 3 : 3 : 1.
Total plants = 1600
(i) Tall with round seeds (9 parts):
$$\frac{9}{16} \times 1600 = \textbf{900 plants}$$
(ii) Short with wrinkled seeds (1 part):
$$\frac{1}{16} \times 1600 = \textbf{100 plants}$$
Conclusion:
Mendel concluded that two traits (height and seed shape) are inherited independently of each other. The F₂ generation shows a 9:3:3:1 ratio, which proves that factors (genes) for different characters assort independently during gamete formation. This is known as the Law of Independent Assortment.
Source: Heredity and Evolution, Mendel's Laws of Inheritance
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Explanation
- The dihybrid cross ratio 9:3:3:1 must be stated clearly; examiners expect students to know it by heart.
- Show the calculation (fraction × 1600) for full method marks.
- The conclusion must name the Law of Independent Assortment — this is the key scoring point for the last part.
- Do not mix up 9:3:3:1 with the 3:1 monohybrid ratio.
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