Why is a dihybrid cross 9:3:3:1?

Each AaBb parent makes four kinds of gamete (AB, Ab, aB, ab) in equal numbers, so the square has 4 x 4 = 16 equally likely cells. For each gene alone the dominant phenotype has probability 3/4 and the recessive 1/4, and because the genes are assumed unlinked the probabilities multiply: 3/4 x 3/4 = 9/16 show both dominant traits, 3/4 x 1/4 = 3/16 each for one dominant and one recessive, and 1/4 x 1/4 = 1/16 show both recessive traits.

How do you do a Punnett square for an X-linked trait?

Write the alleles on the X: a carrier mother is X^H X^h and an unaffected father is X^H Y. The mother's gametes are X^H and X^h, the father's X^H and Y. Daughters get the father's X, so in this cross none of them show a recessive trait, though half are carriers; sons get the father's Y and their only X from the mother, so half of the sons are affected. Read daughters and sons separately: 1/2 of sons, which is 1/4 of all children.

Can two type A parents have a type O child?

Yes, if both are I^A i. Each passes i with probability 1/2, so the chance of ii (type O) is 1/2 x 1/2 = 1/4. Two I^A I^A parents cannot. A blood test shows the type, not the genotype, so this page asks you to pick the genotype and shows both possibilities when you are unsure.

Can a Punnett square tell me my baby's eye colour?

No. Eye colour and hair colour are controlled by many genes, so a one-gene square gives a wrong answer for them; brown-eyed parents can have blue-eyed children and blue-eyed parents can, rarely, have brown-eyed ones. This calculator is for textbook single-gene traits.

Punnett square calculator

Type both parents. Get the square, the genotype and phenotype ratios as exact fractions, and the working.

The cross

Parent 1

Parent 2

Name the traits (optional)

Names show in the key, the ratios and the exports, for example tall and short for T and t.

Examples

Practice sheet

A worksheet of mixed problems: one gene, two and three genes, incomplete dominance, X-linked and blood type. The same seed always gives the same sheet, so a class can share one.

Problems Seed

The square

Parent 1's gametes run across the top, parent 2's down the side; each cell is one equally likely child.

Ratios

Phenotype ratio

Genotype ratio

Probability of

How to use it

Pick the inheritance pattern. Dominant is Mendel's peas: one capital letter is enough to show the trait. Incomplete (incomplete dominance or codominance) gives the heterozygote its own look (red × white snapdragons give pink). X-linked puts the gene on the X chromosome. Blood type uses the ABO alleles, with Rh as an optional second gene.
Type each parent's genotype, two letters per gene: Aa, AaBb, AaBbCc, up to four genes. Or pick one from the list under the box.
Read the square and the ratios. Choose any outcome under Probability of to see the multiplication behind it.

How big the square gets

Each gene a parent is heterozygous for doubles its gametes: one gene gives 2, two give 4, three give 8, four give 16. This page always writes every gamete out, even when two are identical, so the cells stay equally likely and you can count them: 2 × 2 for one gene, 4 × 4 (16 cells) for two, 8 × 8 (64) for three. Four genes need 16 × 16 = 256 cells; that is drawn as a scrolling table, because nobody can read it as a picture, and the ratios underneath are what you want from it.

Worked example: AaBb × AaBb

Gametes of each parent: AB, Ab, aB, ab, each 1/4.
The 4 × 4 square has 16 cells. Count the phenotypes: 9 cells have at least one A and one B, 3 have A but bb, 3 have aa but B, and 1 is aabb. That is the 9 : 3 : 3 : 1 ratio.
You do not need the square for a single outcome. Split it by gene and multiply: P(aabb) = P(aa) × P(bb) = 1/4 × 1/4 = 1/16, and P(A_bb) = 3/4 × 1/4 = 3/16. The page writes this line out for whatever you choose.
The genotypes come out as 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 (AABB, AABb, AAbb, AaBB, AaBb, Aabb, aaBB, aaBb, aabb), nine genotypes behind four phenotypes.

With three genes, AaBbCc × AaBbCc gives 64 cells and a phenotype ratio of 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1; the chance of the all-recessive aabbcc is (1/4)³ = 1/64.

X-linked crosses: daughters and sons are read apart

Write the allele as a superscript on the X: X^HX^h for a carrier mother, X^HY for a father (the ^ is optional; XHXh works). Sons get their only X from their mother, so they show whatever allele she passes. Daughters get one X from each parent. For a carrier mother and an unaffected father, no daughter is affected but half are carriers, and half of the sons are affected: 1/2 of sons, which is 1/4 of all children. That is why haemophilia and red-green colour blindness are far more common in men.

Blood type: ABO and Rh

ABO is one gene with three alleles. I^A and I^B are each dominant over i, and together they are codominant: I^AI^B is type AB. So type A is I^AI^A or I^Ai, type B is I^BI^B or I^Bi, and type O is only ii. Two I^Ai × I^Bi parents can have children of all four types, each with probability 1/4. Rh is a separate gene: the D allele (Rh positive) is dominant, so two Rh-negative (dd) parents have only Rh-negative children. Tick Add Rh to cross both genes in a 4 × 4 square, for example P(O negative) = P(O) × P(Rh−).

Sources for these rules, fetched 2026-10-24: ABO, ABO blood group system (section “Genetics”, https://en.wikipedia.org/wiki/ABO_blood_group_system); Rh, Rh blood group system (section “Inheritance”, https://en.wikipedia.org/wiki/Rh_blood_group_system).

What it cannot do

Textbook single-gene inheritance only. Each gene is one locus with simple dominance, incomplete dominance or codominance.
Eye colour and hair colour are not single-gene traits. They depend on many genes, so a Punnett square for them gives wrong answers: brown-eyed parents can have a blue-eyed child, and the reverse happens too. The same goes for height, skin colour and most real human traits.
No linkage and no epistasis. Genes are assumed to sort independently; genes close together on one chromosome do not, and one gene masking another (such as the 9 : 3 : 4 of coat colour in Labradors) is not modelled.
Not a paternity or medical test. It gives textbook probabilities for genotypes you type. Rare blood types such as Bombay (hh) and cis-AB break the simple ABO rules, which is one reason blood type cannot prove or rule out parentage. For a real family question, see a genetic counsellor.
Only one X-linked gene at a time, and only two alleles per gene except ABO's three.
Nothing is uploaded or saved. The square is worked out on this device; reloading the page starts again.

Questions

What is the difference between genotype ratio and phenotype ratio?

The genotype ratio counts the allele combinations; the phenotype ratio counts what you would see. For Aa × Aa they are 1 AA : 2 Aa : 1 aa and 3 dominant : 1 recessive, because AA and Aa look the same. With incomplete dominance the heterozygote looks different, so both ratios are 1 : 2 : 1.

What is a test cross?

Crossing an organism showing the dominant trait with a homozygous recessive one (A_ × aa). If any offspring show the recessive trait, the parent was Aa, and you expect 1 : 1. Pick Test cross in the examples to see it.

Why do my real results not match the ratio exactly?

A Punnett square gives probabilities, not counts. Each child is a fresh draw, so a family of four from Aa × Aa has all four showing the dominant trait about a third of the time ((3/4)⁴ ≈ 0.32). The ratio appears only over many offspring.