Q.What is a dihybrid cross? Explain it with a suitable example and the checker board (Punnett square) method.

Step 1. Definition. A dihybrid cross is a cross between parents differing in two heritable traits, e.g., a pure round, yellow-seeded pea plant (RRYY) crossed with a pure wrinkled, green-seeded plant (rryy).

Step 2. Parental gametes and F1. The RRYY parent produces only RY gametes and the rryy parent only ry gametes, so every F1 offspring is RrYy — phenotypically Round and Yellow, since both dominant alleles mask their recessive partners.

Step 3. F1 gametes. By the Law of Independent Assortment, each RrYy F1 plant produces four types of gametes in equal proportion during meiosis: RY, Ry, rY and ry.

Step 4. The checker-board (Punnett square). Selfing F1 x F1 means crossing these four gamete types against each other, giving a 4×4=16-box grid. Filling in each box (e.g., RY × RY → RRYY; RY × ry → RrYy; and so on for all 16 combinations) and then reading off phenotypes gives: 9 boxes Round-Yellow, 3 boxes Round-Green, 3 boxes Wrinkled-Yellow, 1 box Wrinkled-Green.

Step 5. Result. The F2 phenotypic ratio is 9:3:3:1, and this specific ratio is only produced because the seed-shape and seed-colour gene pairs assort into gametes independently of one another.

Build the parental cross through F1 gametes into a full 4x4 Punnett square, then tally the 16 boxes into phenotypic classes.

• Forgetting that each F1 plant produces four gamete types, not two, once two gene pairs are involved.
• Miscounting the 16-box grid and arriving at a ratio other than 9:3:3:1.