A pea plant true-breeding for tall stem (TT) is crossed with a true-breeding short plant (tt). In the F2 generation, the expected ratio of TT : Tt : tt plants is 1 : 2 : 1. Describe the cross you would perform to experimentally confirm that the 'tall' plants in F2 are actually a mixture of TT and Tt genotypes rather than all being the same.
Generated by claude-sonnet-4-6 · 2026-06-26 01:03 · grounding rag
Model Answer
Test Cross (Cross with homozygous recessive):
To confirm whether the tall F₂ plants are TT or Tt, each tall F₂ plant is crossed individually with a true-breeding short plant (tt) — this is called a test cross.
Expected results:
- If the tall F₂ plant is TT: all offspring will be tall (Tt).
- If the tall F₂ plant is Tt: offspring will be tall and short in 1:1 ratio (Tt : tt).
By observing the progeny of each cross, we can identify which tall F₂ plants are TT and which are Tt, confirming the expected 1:2 ratio of TT:Tt among the tall plants.
Source: Chapter 8, Heredity
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Explanation
- Examiners expect the term test cross to be named.
- Key point: the tall F₂ plant is crossed with tt (not another tall plant), because tt produces only 't' gametes, so any variation in offspring reveals the unknown genotype.
- State both outcomes clearly — all tall = TT; 1:1 tall:short = Tt. This is where most marks lie.
- No need for a full Punnett square unless specifically asked; stating the ratios is sufficient for 3 marks.