Q1. [3] medium thorough-understanding
When Mendel crossed tall pea plants with short pea plants, all F1 progeny were tall. Yet when these F1 plants were self-pollinated, one-quarter of the F2 progeny were short. What does the reappearance of short plants in F2 prove about what the F1 tall plants were actually carrying?
Generated by claude-sonnet-4-6 · 2026-06-26 01:03 · grounding rag
Model Answer
The reappearance of short plants in F2 proves that the F1 tall plants were not pure for tallness — they were carrying both the tallness factor (T) and the shortness factor (t), i.e., they were Tt (heterozygous). The 't' factor was present but not expressed because 'T' is dominant over 't'. Only when two copies of 't' came together (tt) in F2 did the short trait reappear. This proves that both traits were inherited by F1 plants, but only the dominant trait (tallness) was expressed.
Source: Chapter 8, Section 8.2.2 – Rules for the Inheritance of Traits
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Explanation
- The examiner wants three key ideas: (1) F1 plants were heterozygous (Tt), not pure tall (TT); (2) the recessive 't' factor was hidden/not expressed in F1; (3) F2 self-pollination produced tt offspring, revealing the hidden recessive trait.
- Use terms: dominant, recessive, heterozygous, factors/genes — these fetch marks.
- The textbook explicitly states: "both the tallness and shortness traits were inherited in the F1 plants, but only the tallness trait was expressed." Quote or paraphrase this logic directly.