A pure pea plant having round (R), yellow (Y) seeds is crossed with another pure pea plant having wrinkled (r), green (y) seeds. Subsequently F₁ progeny is self-pollinated to obtain F₂ progeny.
Generated by claude-sonnet-4-6 · 2026-06-15 06:52 · grounding rag
Model Answer
(a) The seeds of F₁ generation will be round and yellow, as roundness (R) and yellowness (Y) are dominant traits.
(b) The four possible combinations in F₂ generation are:
- Round, Yellow
- Round, Green
- Wrinkled, Yellow
- Wrinkled, Green
Their ratio is 9 : 3 : 3 : 1.
(c) The new combinations (Round Green and Wrinkled Yellow) appear because the factors (genes) controlling seed shape and seed colour assort independently during gamete formation and recombine randomly during fertilisation to form the zygote.
Source: Chapter 8, Section 8.2.2
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Explanation
- (a) is straightforward — F₁ of a dihybrid cross always shows both dominant traits (RrYy phenotype).
- (b) Examiners expect all four phenotypes listed clearly with the 9:3:3:1 ratio. Missing the ratio or any combination loses the mark.
- (c) The key phrase the examiner looks for is independent assortment of traits/genes. The textbook specifically states traits are "independently inherited" and new combinations arise when "factors controlling seed shape and seed colour recombine to form the zygote." Use this language.