Mendel crossed pure tall pea plants (TT) with pure short pea plants (tt) and obtained F₁ progeny. When the plants of F₁ progeny were self-pollinated, plants of F₂ progeny were obtained.
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Model Answer
(a) The F₁ progeny plants all looked tall. Their gene combination was Tt (hybrid tall).
(b) In F₁ plants, the gene for shortness (t) could not be expressed because the gene for tallness (T) is dominant over the gene for shortness (t), which is recessive. A single copy of 'T' is sufficient to make the plant tall.
(c) In F₂ progeny, the ratio of tall to short plants was 3 : 1.
Conclusion: Both tallness and shortness traits were inherited by F₁ plants, but only tallness was expressed. Recessive traits are not lost; they reappear in later generations.
Source: Chapter 8, Section 8.2.2 – Rules for the Inheritance of Traits
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Explanation
- (a) Cross TT × tt → all offspring are Tt. Since T is dominant, all appear tall. Examiners expect both the phenotype and genotype.
- (b) The key terms examiners look for are dominant (T) and recessive (t). The textbook explicitly states "a single copy of T is enough to make the plant tall."
- (c) The 3:1 phenotypic ratio (3 tall : 1 short) is a standard fact. The conclusion must mention that the recessive trait is not lost but reappears — this is the heart of Mendel's finding and is directly from the textbook passage.