How to use this calculator
Write each parent’s genotype as pairs of letters, one pair per gene, with uppercase for the dominant allele: Aa for a heterozygote, AA or aa for homozygotes, AaBb for two genes. Up to three genes are supported (a trihybrid square has 64 boxes). The square lists each parent’s gametes along the edges, and the tables underneath give the genotype and phenotype ratios with their probabilities. Recessive-phenotype boxes are shaded.
To work backwards, type an observed phenotype ratio such as 3:1, 1:1 or 9:3:3:1 and choose one or two genes. The calculator lists every pair of parent genotypes that produces that ratio; click one to load it into the square. A 1:1 ratio, for example, points to a test cross Aa × aa.
The formula
- A
- dominant allele
- a
- recessive allele
- A_
- dominant phenotype (AA or Aa)
- aa
- recessive phenotype
Worked example
Mendel crossed pea plants heterozygous for seed color (Yy) and seed shape (Rr). What proportion of the offspring are green and wrinkled?
- Each parent (RrYy) makes four gamete types: RY, Ry, rY and ry, giving a 4 × 4 square of 16 boxes.
- Phenotypes: 9 R_Y_ (round yellow) : 3 R_yy : 3 rrY_ : 1 rryy.
- Green and wrinkled (rryy) = 1/16 = 6.25%.
The classic 9:3:3:1 dihybrid ratio — load it with the “Load example” button.
Classic Mendelian ratios
| Cross | Phenotype ratio | Genotype ratio |
|---|---|---|
| Aa × Aa | 3 : 1 | 1 AA : 2 Aa : 1 aa |
| Aa × aa (test cross) | 1 : 1 | 1 Aa : 1 aa |
| AA × aa | all dominant | all Aa |
| AaBb × AaBb | 9 : 3 : 3 : 1 | 9 genotypes, 1:2:1:2:4:2:1:2:1 |
| AaBb × aabb | 1 : 1 : 1 : 1 | 1 : 1 : 1 : 1 |
Assumptions and limits
- Complete dominance at every gene. Incomplete dominance or codominance (e.g. ABO blood groups) gives different phenotype ratios — read the genotype table instead.
- Genes assort independently (no linkage), and all genotypes are equally viable.
- Ratios are expectations; small families or litters often deviate by chance.
Frequently asked questions
What is the probability of a recessive phenotype from two carriers?
1 in 4 (25%). The Aa × Aa square has one aa box out of four.
How many boxes does a dihybrid Punnett square have?
Sixteen: each AaBb parent makes four gamete types, and 4 × 4 = 16.
What does a 1:1 ratio tell me?
For one gene with complete dominance, it means one parent is heterozygous and the other homozygous recessive — the basis of the test cross.
Sources
- OpenStax Biology 2e, §12.2 Characteristics and Traits (Punnett squares)
- OpenStax Biology 2e, §12.3 Laws of Inheritance
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