How to use
- Type the genotype of each parent, one letter pair per gene: a capital letter is the dominant allele, a small one the recessive allele (Aa, AaBb, AaBbCc).
- Both parents must have the same genes; their order does not matter (BbAa is read as AaBb).
- Read the phenotype ratio at the top, then the square itself and the tables with the share of each phenotype and genotype.
How it works
Each parent passes one allele of every gene to a gamete. For n genes a parent has 2ⁿ gamete combinations (2, 4 or 8), listed with the first gene changing slowest: AaBb gives AB, Ab, aB, ab. The square combines every gamete of parent 1 (rows) with every gamete of parent 2 (columns), so all cells are equally likely. Counting identical cells gives the genotype ratio; grouping them by phenotype — A_ when at least one dominant allele is present, aa otherwise — gives the phenotype ratio. Ratios are reduced by their common divisor.
Worked example
AaBb × AaBb: 4 × 4 = 16 cells. The phenotypes are A_B_ 9/16 (56.25 %), A_bb 3/16 (18.75 %), aaB_ 3/16 (18.75 %) and aabb 1/16 (6.25 %), so the phenotype ratio is 9 : 3 : 3 : 1. There are nine genotypes in the ratio 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 — the most common one is AaBb with 4/16 (25 %).
A monohybrid cross Aa × Aa gives AA 1/4, Aa 2/4, aa 1/4 (1 : 2 : 1) and the phenotype ratio 3 : 1.
Tips
- A homozygous parent such as aabb still gets a full row of gametes (four identical ab), so the square keeps 16 cells and the shares stay right.
- Use different letters for different genes, not A and a for one gene and A for another trait.
- Three genes give an 8 × 8 square of 64 cells and the classic 27 : 9 : 9 : 3 : 9 : 3 : 3 : 1 phenotype ratio for AaBbCc × AaBbCc.