Mendel crossed pea plants having tall stems and round seeds (TTRR) with plants having short stems and wrinkled seeds (ttrr). All F1 plants were tall with round seeds. When F1 plants were allowed to self-pollinate:
(i) What phenotypic ratio is expected in F2?
(ii) Identify the two new phenotypic combinations that appear in F2 which were absent in the parent generation.
(iii) What does the appearance of new trait combinations in F2 indicate about how traits are inherited?
Generated by claude-sonnet-4-6 · 2026-06-26 01:03 · grounding rag
Model Answer
(i) The expected phenotypic ratio in F2 is 9 : 3 : 3 : 1
(Tall round : Tall wrinkled : Short round : Short wrinkled)
(ii) The two new phenotypic combinations appearing in F2 are:
- Tall plants with wrinkled seeds
- Short plants with round seeds
(iii) The appearance of new trait combinations in F2 indicates that the two traits (stem height and seed shape) are inherited independently of each other. The factors (genes) controlling different traits recombine independently during reproduction, giving rise to new combinations in offspring.
Source: Chapter 8, Section 8.2.2 — Rules for the Inheritance of Traits
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Explanation
- Part (i): A dihybrid cross always gives 9:3:3:1 in F2 — memorise this ratio.
- Part (ii): The two parental types were tall-round and short-wrinkled; so the new combinations are tall-wrinkled and short-round. Examiners specifically look for this distinction.
- Part (iii): The key phrase the examiner expects is "independently inherited" (or independent assortment). The textbook explicitly states: "the tall/short trait and the round seed/wrinkled seed trait are independently inherited." Always link the observation (new combinations) to the conclusion (independent inheritance of traits).