Read the following scenario and answer the questions:
Rajat performed a genetics experiment with pea plants. He crossed a plant with round yellow seeds with another plant of identical phenotype. After obtaining a large number of seeds, he found four phenotypic classes: round yellow, round green, wrinkled yellow, and wrinkled green in the approximate ratio 9:3:3:1.
(i) What were the genotypes of both parent plants? Justify your answer based on the phenotypic ratio obtained. (1 mark)
(ii) Out of 160 seeds, how many would Rajat expect to have wrinkled green seeds? (1 mark)
(iii) Which two phenotypic classes in Rajat's results represent new combinations not seen in either parent? What does their appearance indicate about how seed shape and seed colour are inherited relative to each other? (1 mark)
(iv) Rajat's teacher says the 9:3:3:1 ratio would collapse to a 3:1 ratio if the two traits were always inherited together. Explain why this would happen. (1 mark)
Generated by claude-sonnet-4-6 · 2026-06-26 01:04 · grounding rag
Model Answer
(i) Both parents were RrYy (dihybrid). The 9:3:3:1 ratio is the classic F2 dihybrid ratio, which only appears when both parents are heterozygous for both traits (round = R dominant, yellow = Y dominant). Since both parents had identical phenotype (round yellow) yet produced all four phenotypic classes, each must have carried one dominant and one recessive allele for each gene.
(ii) Wrinkled green (rryy) = 1/16 of offspring.
Expected = (1/16) × 160 = 10 seeds
(iii) Round green and wrinkled yellow are new combinations not seen in either parent. Their appearance indicates that seed shape and seed colour are inherited independently of each other (independent assortment), allowing new combinations to arise in offspring.
(iv) If both traits were always inherited together, each parent would pass only one combined unit (e.g., RY or ry together) — effectively behaving as a single trait. The cross would reduce to Aa × Aa, giving only dominant : recessive = 3:1, with no new trait combinations possible.
Source: Chapter 8, Section 8.2.2 — Rules for the Inheritance of Traits
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Explanation
- (i) The 9:3:3:1 ratio is the definitive evidence for a dihybrid cross (RrYy × RrYy). Examiners want you to state the genotype AND justify it using the ratio.
- (ii) Straightforward calculation: wrinkled green = rryy = 1 part out of 16. Always show working.
- (iii) "New combinations" means recombinant phenotypes not present in parents. Link directly to independent inheritance/assortment — the key Mendelian principle the textbook states.
- (iv) Linked traits behave as one gene, so dihybrid becomes monohybrid → 3:1. The teacher's statement is testing whether you understand why independent assortment produces four classes instead of two.