How to read a Punnett square for two traits

A dihybrid cross examines inheritance of two genes that assort independently. Each heterozygous parent (AaBb) can produce four different gametes: AB, Ab, aB, and ab. The Punnett square therefore has four columns and four rows, giving sixteen cells that represent every possible combination of the parental gametes. By examining each cell you can determine the genotype of each offspring and then translate those genotypes into phenotypes.

Step‑by‑step construction

Fill the dihybrid Punnett square by following these steps:

1. 1Write the four possible gametes for each parent across the top and down the side.
2. 2Combine the gamete from the top with the one from the side in each cell.
3. 3Record the resulting genotype in each cell, keeping allele order consistent (e.g., A before a, B before b).

After the grid is complete, group the genotypes by phenotype. For independent assortment, the classic 9:3:3:1 phenotypic ratio emerges when both parents are heterozygous for both traits. This ratio means nine cells show both dominant traits, three show dominant A with recessive b, three show recessive a with dominant B, and one shows both recessive traits. The ratio is derived directly from counting the appropriate cells in the completed square.

The possible gametes each parent can produce are:

• AB
• Ab
• aB
• ab

Worked example

Consider a cross AaBb × AaBb. List the gametes as AB, Ab, aB, ab for both parents and draw a 4×4 grid. Filling the grid yields sixteen genotypes: four A_B_, four A_bb, four aaB_, and four aabb. The genotype A_B_ (both dominant) appears in nine cells because AB×AB, AB×Ab, AB×aB, Ab×AB, Ab×aB, aB×AB, aB×Ab, and aB×aB each contribute. The recessive‑recessive genotype aabb appears only in the single cell where ab×ab meet, giving a fraction of 1/16.

Phenotypic ratios derived from the genotypes: