Consider a dihybrid cross between two heterozygous pea plants for seed shape (R = round, r = wrinkled) and seed colour (Y = yellow, y = green). Both parents have genotype RrYy. Assuming independent assortment, what is the phenotypic ratio of the F1 generation?
Answer
9 : 3 : 3 : 1
The correct answer is 9 : 3 : 3 : 1 — Thus the phenotypic ratio of the F1 generation is 9 : 3 : 3 : 1.
Step-by-step solution
- List the possible gametes from a RrYy parent: RY, Ry, rY, ry.
- Set up a 4 × 4 Punnett square using these gametes for both parents.
- Count the phenotypic classes: round‑yellow (RY × RY, RY × Ry, …) gives 9 squares; round‑green (RY × ry, Ry × RY, …) gives 3 squares; wrinkled‑yellow (rY × RY, …) gives 3 squares; wrinkled‑green (ry × ry, …) gives 1 square.
- Express the numbers as a ratio: 9 : 3 : 3 : 1.
- Thus the phenotypic ratio of the F1 generation is 9 : 3 : 3 : 1.
Common mistake: Giving a 3:1 ratio by treating the two traits as a single gene.
Concept tested: Dihybrid cross – independent assortment
Published · How this solution was prepared and checked
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