Mendel used garden pea plants to study heredity over two generations.
(a) Mendel crossed a pure tall pea plant with a pure short pea plant. With a labelled cross diagram, show the genotypes of parents, F1 and F2 generations. (2 marks)
(b) In the F2 generation, what fraction of the tall plants would be pure-breeding (TT)? What simple experiment would you perform to confirm this? (2 marks)
(c) Why did Mendel choose to count individuals in each generation rather than simply observe traits qualitatively? What advantage did this give? (1 mark)
Generated by claude-sonnet-4-6 · 2026-06-26 01:03 · grounding rag
Model Answer
(a) Cross Diagram – Tall × Short
| Generation | Cross |
|---|---|
| Parents (P) | TT (pure tall) × tt (pure short) |
| F₁ | All Tt (tall) |
| F₁ × F₁ | Tt × Tt |
| F₂ | TT : Tt : tt = 1 : 2 : 1 |
F₂ phenotype ratio → 3 tall : 1 short (TT and Tt are tall; tt is short).
(b) Fraction and Confirmatory Experiment
In F₂, the ratio of TT : Tt : tt is 1 : 2 : 1. Out of 3 tall plants, only 1/3 are pure-breeding (TT).
To confirm: allow each F₂ tall plant to self-pollinate (self-breed) and observe the progeny. A plant that produces only tall offspring in all subsequent generations is TT (pure-breeding), whereas one that gives 3 tall : 1 short is Tt.
(c) Advantage of Counting
Mendel was the first to count the number of individuals showing each trait in every generation rather than just noting traits qualitatively. This gave him exact ratios (3:1 in F₂), which revealed the mathematical pattern of inheritance and allowed him to deduce the laws governing heredity.
Source: Chapter 8 – Heredity, Section 8.2.2 Rules for the Inheritance of Traits – Mendel's Contributions
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Explanation
- (a) Draw the cross as a table or Punnett-square-style layout. Label clearly: P generation genotypes (TT, tt), F₁ (Tt), F₁ × F₁, and F₂ (TT, Tt, Tt, tt). Examiners award marks for correct genotypes AND the ratio.
- (b) The key trap: students often say "1/4 of all F₂ are TT," which is correct, but the question asks what fraction of the tall plants are TT. Among 3 tall plants (TT + Tt + Tt), only 1 is TT → 1/3. The self-pollination experiment is directly from Activity 8.2 in the textbook.
- (c) The textbook explicitly states Mendel "was the first one to keep count of individuals exhibiting a particular trait in each generation. This helped him to arrive at the laws of inheritance." Use this language.