Tackling Dihybrid Crosses—How Independent Assortment Sank In

 Moving on to dihybrid inheritance, I faced two traits at once. I picked seed shape (round R vs wrinkled r) and seed color (yellow Y vs green y). Sketching a 4×4 Punnett square felt tedious, so I simplified it by first listing the four gamete types for an RrYy parent: RY, Ry, rY, ry. Then I paired those with the other parent’s gametes. Filling in 16 boxes gave me the familiar 9 : 3 : 3 : 1 phenotypic ratio.

I called this Mendel’s second law—the law of independent assortment. To verify, I actually flipped coins (heads = R or Y, tails = r or y) to simulate gamete formation. After dozens of flips, the ratios hovered close to expectations. That hands-on practice made the abstract law feel real.

Sample SPM Question
“Two heterozygous corn plants (AaBb) are crossed. Using a Punnett square, calculate the proportion of offspring showing both dominant phenotypes.”

My answer steps:

1. List gametes: AB, Ab, aB, ab.
2. Draw 4×4 square and fill.
3. Count boxes with A_B_ (both dominant)—9 out of 16 or 56.25 %.