Punnett Square Guide — Monohybrid and Dihybrid Crosses Explained

Common Cross Types and Ratios

How to Use the Punnett Square Generator

1. Open the Punnett square generator.
2. Enter Parent 1 genotype: Aa for heterozygous, AA for homozygous dominant, aa for homozygous recessive.
3. Enter Parent 2 genotype in the same format.
4. Optionally name the dominant and recessive phenotypes (e.g. Purple / White).
5. The grid, genotype ratio, and phenotype ratio appear instantly.

Reading the Punnett Square

The grid layout

The top row shows the gametes of one parent; the left column shows the gametes of the other. Each cell in the grid is one possible offspring, formed by combining one gamete from each parent. For an Aa × Aa cross, the four cells give AA, Aa, Aa, aa — a 1:2:1 genotype ratio.

Colour coding

Green cells are homozygous dominant (AA), yellow are heterozygous (Aa), and red are homozygous recessive (aa). The colour distribution immediately shows the phenotype ratio — in Aa × Aa, green + yellow (3) versus red (1) gives the classic 3:1 ratio.

Dihybrid Crosses and Independent Assortment

A dihybrid cross tracks two genes simultaneously. Each parent with genotype AaBb produces four types of gametes: AB, Ab, aB, and ab (following Mendel's law of independent assortment). The 4×4 Punnett square has 16 cells. For AaBb × AaBb, the phenotype ratio is 9:3:3:1 — 9 dominant for both traits, 3 dominant for trait A only, 3 dominant for trait B only, and 1 recessive for both.

Test Crosses

A test cross crosses an individual showing the dominant phenotype with a homozygous recessive (aa). If all offspring show the dominant phenotype, the tested parent is AA. If half show the recessive phenotype, the parent is Aa. Enter the unknown parent and aa to run the test cross instantly.

Common Questions

What does heterozygous mean?

Heterozygous means the two alleles for a gene are different — one dominant (A) and one recessive (a). Heterozygous individuals (Aa) show the dominant phenotype but are carriers of the recessive allele, which can reappear in offspring if they mate with another carrier.

What is incomplete dominance?

In incomplete dominance, neither allele is fully dominant. A cross between red (RR) and white (rr) flowers gives pink (Rr) offspring. The Punnett square ratios still apply, but all three genotypes show distinct phenotypes (red, pink, white) rather than two (dominant and recessive). Name the heterozygous phenotype accordingly in the trait fields.