Mendel’s Experiments and Laws – Long Answer Questions (CBSE Class 10 Science)

Medium Level (Application & Explanation)

Q1. Why did Mendel choose pea plants for his experiments, and how did these choices make his conclusions reliable?

Answer: Mendel chose pea plants (Pisum sativum) because they have clear contrasting traits such as tall vs dwarf, round vs wrinkled seeds, and yellow vs green seeds, which made differences easy to observe. Pea flowers generally self-pollinate, allowing him to maintain pure lines like TT (pure tall) and tt (pure dwarf). He could also control cross-pollination by removing stamens and applying pollen manually, ensuring specific crosses like TT × tt. Pea plants have a short life cycle and produce many offspring, giving large sample sizes that made ratios like 3:1 and 9:3:3:1 visible and reliable. These features ensured controlled conditions, repeatable results, and clear data, allowing Mendel to frame the Law of Dominance, Law of Segregation, and Law of Independent Assortment with confidence.

Q2. Describe a monohybrid cross for plant height and explain how it demonstrates the Law of Dominance.

Answer: In a classic monohybrid cross, Mendel crossed a pure tall pea plant (TT) with a pure dwarf plant (tt). All F₁ generation plants were tall (Tt) because the tall allele (T) is dominant and masks the dwarf allele (t). When the F₁ plants (Tt) were self-pollinated (Tt × Tt), the F₂ generation showed two types of plants: tall (TT, Tt) and dwarf (tt) in a 3 tall : 1 dwarf ratio. This demonstrates the Law of Dominance, which states that in a pair of contrasting traits, one trait is dominant and masks the other recessive trait in the F₁. The reappearance of dwarf in F₂ shows that the recessive allele was present but hidden in F₁, not lost.

Q3. Using Tt × Tt for plant height, explain the Law of Segregation and the resulting F₂ ratio.

Answer: Consider two F₁ tall plants with genotype Tt. During gamete formation, the two alleles separate (segregate), so each plant forms two types of gametes: T and t. When these gametes fuse randomly (T × T, T × t, t × T, t × t), the F₂ genotypes formed are TT, Tt, tT, tt. Phenotypically, this becomes 3 tall (TT, Tt, tT) and 1 dwarf (tt), giving the classic 3:1 ratio. This illustrates the Law of Segregation (Purity of Gametes): each individual carries two alleles, but each gamete receives only one. Thus, gametes are pure for a single allele (either T or t), and alleles do not blend. The reappearance of the recessive dwarf trait confirms that the alleles separate and recombine in offspring.

Q4. Explain a dihybrid cross for seed color and shape, and show how it supports the Law of Independent Assortment.

Answer: Mendel crossed plants with yellow, round seeds (YYRR) and green, wrinkled seeds (yyrr). All F₁ plants had yellow, round seeds (YyRr) because yellow (Y) and round (R) are dominant over green (y) and wrinkled (r). When F₁ plants (YyRr × YyRr) were self-pollinated, the F₂ generation showed four combinations in a 9:3:3:1 ratio: 9 yellow round, 3 yellow wrinkled, 3 green round, 1 green wrinkled. This supports the Law of Independent Assortment: the inheritance of seed color and seed shape occurs independently, so alleles for color (Y/y) assort separately from alleles for shape (R/r). The presence of new combinations like yellow wrinkled and green round in specific proportions confirms that traits are not always inherited together, but assort independently.

Q5. Outline Mendel’s method to perform a controlled cross in pea plants and explain why each step was important.

Answer: Mendel’s method ensured accurate and controlled results:

He selected pure-breeding parents (e.g., TT and tt) to start with stable traits.
He removed stamens (male parts) from the flower of the plant chosen to be the female parent, preventing self-pollination.
He collected pollen from the male parent and manually dusted it onto the stigma of the prepared flower to achieve a specific cross.
He bagged or covered the pollinated flowers to prevent unwanted pollen from entering.
He collected seeds (F₁), planted them, recorded phenotypes, and then self-pollinated F₁ to get F₂. Each step maintained experimental control, prevented contamination, and provided reliable phenotypic ratios like 3:1 and 9:3:3:1, which were the basis for the Law of Dominance, Law of Segregation, and Law of Independent Assortment.

High Complexity (Analytical & Scenario-Based)

Q6. Two tall pea plants are crossed, and some F₁ offspring are dwarf. Deduce the likely genotypes of the parents and justify your reasoning.

Answer: If tall × tall gives some dwarf offspring, both parents must carry the recessive allele (t). The most likely parental genotypes are Tt × Tt. Here is why:

If either parent were TT, none of the offspring could be tt (dwarf), because they would always receive at least one T.
In a Tt × Tt cross, gametes are T and t from each parent. The F₁ genotypes become TT, Tt, tT, tt, making 25% dwarf (tt) and 75% tall (TT or Tt) on average.
The presence of dwarf (tt) confirms the Law of Segregation, showing that alleles separate and recombine to reveal recessive traits when both recessive alleles come together. Thus, both parents are heterozygous tall (Tt), not pure tall.

Q7. In an F₂ dihybrid cross for seed color and shape, a student counts 178 yellow round, 60 yellow wrinkled, 55 green round, and 12 green wrinkled seeds. Analyze whether this supports independent assortment.

Answer: The ideal 9:3:3:1 ratio in a dihybrid F₂ can be compared to the observed numbers:

Total seeds = 178 + 60 + 55 + 12 = 305
Expected proportions: 9/16, 3/16, 3/16, 1/16
Approximate expected counts:
- Yellow round ≈ 171
- Yellow wrinkled ≈ 57
- Green round ≈ 57
- Green wrinkled ≈ 19 The observed values (178, 60, 55, 12) are close to expectations, with small deviations likely due to chance and sampling error. Mendelian ratios are probabilistic, not exact, especially with moderate sample sizes. The presence of all four phenotypic classes in roughly 9:3:3:1 proportions supports the Law of Independent Assortment. Minor differences can arise from random segregation, counting variation, or sample size, not necessarily a failure of independence.

Q8. A plant with genotype YyRr is crossed with yyrr. Predict the F₁ phenotypes and their expected ratio. Explain your logic using independent assortment.

Answer: Cross: YyRr × yyrr. The YyRr parent forms four gametes with equal probability due to independent assortment: YR, Yr, yR, yr. The yyrr parent can only form yr gametes. The possible F₁ offspring are:

YR × yr → YyRr (yellow, round)
Yr × yr → Yyrr (yellow, wrinkled)
yR × yr → yyRr (green, round)
yr × yr → yyrr (green, wrinkled) Each combination is equally likely, so the expected phenotypic ratio is 1:1:1:1 for:
Yellow round
Yellow wrinkled
Green round
Green wrinkled This outcome illustrates the Law of Independent Assortment, as alleles for color (Y/y) and shape (R/r) are inherited separately, creating all combinations in equal frequency when crossed with a double recessive.

Q9. A farmer wants to ensure only yellow-seeded pea plants in future crops. Using Mendel’s laws, suggest a reliable breeding strategy and justify it.

Answer: To ensure only yellow seeds, the farmer must develop and use pure-breeding yellow (YY) lines:

First, identify yellow plants that may be YY or Yy, since yellow (Y) is dominant over green (y).
Self-pollinate each yellow plant and observe its progeny:
- If any green (yy) seeds appear, the parent was Yy.
- If all seeds remain yellow for multiple generations, the parent is likely YY.
Maintain and multiply only the true-breeding YY plants. This strategy uses the Law of Dominance (yellow masks green in Yy) and the Law of Segregation (Y and y separate in gametes). By selecting YY, no y allele is present, so green seeds (yy) cannot reappear. Regular selfing and selection stabilize the trait in the population.

Q10. If two traits were not inherited independently, how would F₂ results differ from Mendel’s 9:3:3:1 ratio? Use a thought experiment to evaluate Mendel’s conclusion.

Answer: If traits did not assort independently, certain combinations would occur together more often, and the F₂ ratio would deviate from 9:3:3:1. For example, imagine yellow always occurred with round, and green always with wrinkled (i.e., traits “travel together”). In such a case:

You would mostly see yellow round and green wrinkled, with very few or no yellow wrinkled or green round.
The presence of significant numbers of yellow wrinkled and green round in Mendel’s F₂ proved that color and shape could recombine freely. Thus, Mendel’s observation of all four phenotypes in a 9:3:3:1 proportion supported the Law of Independent Assortment. The thought experiment highlights that without independence, new combinations would be rare or absent, contradicting Mendel’s actual results.