Q1. [3] medium thorough-understanding
In a dihybrid cross, an F1 plant with genotype RrYy is self-pollinated. Among the F2 seeds, new combinations appear that were not present in either parent. Explain why offspring with round, green seeds (Rryy) can appear even though neither parent carried that combination, and state which principle of inheritance this demonstrates.
Generated by claude-sonnet-4-6 · 2026-06-26 01:03 · grounding rag
Model Answer
In a dihybrid cross, RrYy (round, yellow) parent produces gametes by independent segregation of alleles. The R allele (round) and y allele (green) can combine in a gamete as Ry, and fertilisation can produce Rryy offspring. Although neither parent had round + green together, the alleles for each trait were present separately and can recombine independently during gamete formation.
This demonstrates Mendel's Law of Independent Assortment — factors controlling different traits (seed shape and seed colour) are inherited independently of each other, allowing new combinations to appear in F2 progeny.
Source: Chapter 8, Section 8.2.2
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Explanation
- Examiners expect you to mention gamete formation (specifically that alleles segregate independently) and name the law explicitly.
- The key phrase from the textbook is: "factors controlling for seed shape and seed colour recombine to form zygote leading to F2 offspring" — use this idea.
- Do NOT confuse this with the Law of Segregation (which is about two alleles of the same gene separating). Independent Assortment is about different genes behaving independently.
- For 3 marks, cover: (1) how Ry gametes are formed, (2) recombination at fertilisation gives Rryy, (3) name of the principle.