In pea plants, round seed shape (R) is dominant over wrinkled (r). A plant with round seeds is test-crossed with a wrinkled-seeded plant and gives both round and wrinkled seeds in equal proportion. What is the genotype of the round-seeded parent? Justify your answer.
Generated by claude-sonnet-4-6 · 2026-06-26 01:03 · grounding rag
Model Answer
The genotype of the round-seeded parent is Rr (heterozygous).
Justification: A test cross is performed between the plant of unknown genotype and a homozygous recessive (rr) plant. The cross Rr × rr produces Rr (round) and rr (wrinkled) in a 1:1 ratio, which matches the equal proportion observed. If the parent were RR, all offspring would be round (Rr only).
Source: Heredity, Chapter 8, Section 8.2.2 & 8.2.3
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Explanation
- Test cross always involves crossing with the homozygous recessive (rr). The 1:1 output of round : wrinkled is the classic result of a heterozygous × homozygous recessive cross.
- Examiners expect you to (1) state the genotype clearly, (2) show or describe the cross, and (3) explain why the other possibility (RR) is ruled out. All three points fit in the word budget for 2 marks.
- Write the cross explicitly (Rr × rr → Rr : rr) — it earns the second mark.