What your result means
The square shows every equally likely combination of one allele from each parent, so each cell is one outcome of the same probability. The genotype ratio counts the allele pairs; the phenotype ratio counts what you would actually see, which is coarser because a dominant allele masks a recessive one. That is why a 1:2:1 genotype ratio appears as a 3:1 phenotype ratio: the homozygous dominant and the heterozygous offspring look identical.
Why this one is different
You name the traits, alleles and phenotypes, so the square matches the question in front of you rather than a generic A and a. Monohybrid and dihybrid crosses both work, with genotype and phenotype ratios and a step-through that builds the grid one gamete pairing at a time.
How it works
Each parent carries two alleles for a trait and passes exactly one to each offspring, chosen at random — Mendel's law of segregation. Write parent 1's possible gametes across the top and parent 2's down the side, then fill each cell with the pair those two gametes make. A monohybrid cross has two gametes per parent, so four cells. A dihybrid cross tracks two genes at once: each parent makes four gamete types because the two genes assort independently, giving a 4×4 grid of sixteen equally likely cells.
How to use this calculator
- Pick monohybrid for one trait or dihybrid for two.
- Name the trait and choose its allele letter. The capital is the dominant allele, the lower case the recessive one.
- Name what each phenotype looks like, for example Tall and Short.
- Set each parent's genotype: homozygous dominant, heterozygous, or homozygous recessive.
- Read the grid and the two ratios. Share the link to hand the exact cross to someone else.
Formula
Monohybrid total cells = 2 × 2 = 4
Dihybrid total cells = 4 × 4 = 16
Gametes — the single alleles a parent can pass on: TT gives T and T, Tt gives T and t, tt gives t and t. Homozygous — two identical alleles (TT or tt). Heterozygous — two different alleles (Tt), which under complete dominance shows the dominant phenotype and makes the organism a carrier of the recessive allele. Dominant — one copy is enough to show the trait. Recessive — needs two copies.
Example calculation
Parent 2 gametes: T, t
Cells: TT, Tt, Tt, tt
Genotype ratio: 1 TT : 2 Tt : 1 tt
Phenotype ratio: 3 tall : 1 short — 75% tall, 25% short
Frequently asked questions
Does a 3 to 1 ratio mean three of my four children will show the dominant trait?
No. A Punnett square gives the probability for each offspring independently. Every child of a Tt x Tt cross has a 75% chance of the dominant phenotype and a 25% chance of the recessive one, so four children could easily be 4 and 0, or 2 and 2. The ratio is what you expect on average across many offspring, not a quota.
What is the difference between genotype and phenotype?
The genotype is the allele pair a organism carries, such as TT, Tt or tt. The phenotype is the visible characteristic that results. Under complete dominance TT and Tt give the same phenotype, which is why a 1:2:1 genotype ratio shows up as a 3:1 phenotype ratio.
Can a Punnett square handle blood groups, eye colour or height?
Not with this model. A square like this assumes one gene with two alleles and complete dominance. Human blood group involves three alleles and codominance, and eye colour and height are polygenic, so a 2x2 grid cannot represent them. Use this tool for textbook single-gene traits.
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Assumptions & limitations
This is the textbook Mendelian model, and it is deliberately simple:
- Complete dominance only. No codominance (both alleles showing), no incomplete dominance (a blended phenotype), no multiple alleles as in ABO blood group.
- Autosomal genes. Sex-linked inheritance needs X and Y in the square, which this tool does not model.
- Independent assortment. The dihybrid grid assumes the two genes sit on different chromosomes. Linked genes give ratios that deviate from 9:3:3:1.
- Probabilities, not counts. Ratios describe expected proportions over many offspring; small families routinely depart from them.
- One gene, one trait. Most real characteristics — height, skin colour, most disease risk — are polygenic and also environmental.
- Educational tool. It is not genetic counselling or a clinical risk assessment.