State the law of independent assortment and explain it with a dihybrid cross up to F2.

1. 1 · State the law. When two pairs of traits are combined in a hybrid, segregation of one pair is independent of the other pair.
2. 2 · Set up the cross. P: round yellow (RRYY) × wrinkled green (rryy); gametes RY and ry.
3. 3 · Give the F1. RrYy, all round yellow, since round and yellow are dominant.
4. 4 · List the F1 gametes. RY, Ry, rY and ry in equal numbers; this is independent assortment at work.
5. 5 · Draw the 4 × 4 Punnett square. [Diagram: 16 boxes with genotypes; phenotype classes marked]
6. 6 · Give the ratio and new combinations. 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green; round green and wrinkled yellow are new.
7. 7 · Link to real life. Breeders use this to bring two useful traits, like disease resistance and high yield, into one plant.
8. 8 · Close in one line. So each gene pair behaves on its own, and 9 : 3 : 3 : 1 is simply 3 : 1 × 3 : 1.

Scoring tip: List the four F1 gametes clearly before you draw the grid. Examiners look for 9 : 3 : 3 : 1 and the two new combinations.