Two traits — seed shape (round R, wrinkled r) and seed colour (yellow Y, green y) — are controlled by genes on separate chromosomes. A plant with genotype RRyy is crossed with a plant of genotype rrYY. All F1 plants are round and yellow. When F1 plants are self-pollinated, what fraction of the F2 seeds will be wrinkled and yellow? Show your reasoning.
Generated by claude-sonnet-4-6 · 2026-06-26 01:03 · grounding rag
Model Answer
P cross: RRyy × rrYY → F₁: all RrYy (round, yellow)
F₁ self-pollination: RrYy × RrYy
For seed shape: Rr × Rr → 1 RR : 2 Rr : 1 rr → wrinkled (rr) = 1/4
For seed colour: Yy × Yy → 1 YY : 2 Yy : 1 yy → yellow (Y_) = 3/4
Since the genes are on separate chromosomes, they assort independently:
Fraction of wrinkled and yellow = 1/4 × 3/4 = 3/16
This matches the 3 wrinkled-yellow seeds seen in the 9:3:3:1 F₂ ratio.
Source: Chapter 8, Section 8.2.3; Figure 8.5
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Explanation
- Examiners expect you to show each trait analysed separately, then multiply the fractions — this demonstrates understanding of the Law of Independent Assortment.
- The Punnett square in Fig. 8.5 confirms the 9:3:3:1 ratio; wrinkled-yellow is the second 3, i.e., 3/16.
- Always state the final fraction clearly; writing only "3 out of 16" without working loses a mark.
- The key phrase to include: "genes on separate chromosomes assort independently."