Explain Mendel's dihybrid cross with a checkerboard and state the law it proves.

1. 1 · Define. A dihybrid cross is a cross between parents that differ in two pairs of contrasting characters.
2. 2 · Parents and F₁.
   • P: round yellow (RRYY) × wrinkled green (rryy).
   • Gametes: RY and ry.
   • F₁: all RrYy, round yellow.
3. 3 · F₁ gametes. RrYy forms RY, Ry, rY and ry in equal numbers.
4. 4 · Draw the 16-box checkerboard. [Punnett square: RY, Ry, rY, ry across the top and down the side; 16 genotypes inside]
5. 5 · Results. Phenotypic ratio: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
6. 6 · State the law. Law of independent assortment: alleles of different characters go into gametes independently; the new combinations prove it.
7. 7 · Link to daily life. Breeders combine high yield and disease resistance in one rice variety.
8. 8 · Close. So each character is inherited on its own: 3 : 1 for each, and 9 : 3 : 3 : 1 together.

Scoring tip: Draw the 16 boxes with a ruler and write the gametes along both edges first. A neat checkerboard is half the marks.