Punnett Square Calculator

How to Use the Punnett Square Calculator

1. 1. Pick the cross. Choose monohybrid (one gene), dihybrid (two) or trihybrid (three). Tick Gene 1 is sex-linked if the first gene sits on the X chromosome (or the Z in birds); parent 1 is then the female and parent 2 the male.
2. 2. Set each gene. Give every gene its own letter — the capital is the dominant allele — choose how it is inherited, and pick each parent's genotype from the list.
3. 3. Read the square. The gametes appear down the side and across the top, and each box shows the offspring genotype, tinted by phenotype.
4. 4. Use the tables. Every genotype and phenotype is listed with its number of boxes, its fraction, its percentage probability and the simplified ratio. Open Name the phenotypes to replace the defaults with words such as round and wrinkled.

How a Punnett Square Works

A Punnett square lays out every way two parents' gametes can combine. Each parent produces gametes that carry one allele per gene; the rows hold one parent's distinct gametes and the columns the other's, so each box is one fertilisation. This tool lists distinct gametes only, which means every box has the same probability, 1 ÷ (rows × columns). It assumes the genes are unlinked and assort independently, as Mendel's second law describes.

The pattern is multiplicative: with n heterozygous genes there are 2ⁿ gametes, 4ⁿ boxes, 3ⁿ genotypes and 2ⁿ phenotypes, and the chance of showing the dominant form of every gene is (3/4)ⁿ. For the trihybrid cross that is 27/64, or 42.19%, and the chance of the triple recessive is 1/64, or 1.56%. In the tables the phenotype rows follow the order of the genes, so the trihybrid ratio reads 27 : 9 : 9 : 3 : 9 : 3 : 3 : 1; the line largest first gives the textbook form.

Dominance Options

• Complete dominance. The heterozygote looks like the dominant homozygote, so Aa × Aa gives 3 dominant : 1 recessive.
• Incomplete dominance. The heterozygote is intermediate — a red snapdragon crossed with a white one gives pink — so Aa × Aa gives 1 : 2 : 1 in phenotype as well as genotype.
• Codominance. The heterozygote shows both alleles fully, as in roan cattle or the AB blood type, again giving 1 : 2 : 1. This tool handles two-allele codominance; a system with three alleles, such as ABO, needs a hand-drawn square.

Sex-Linked Crosses

A gene on the X chromosome is carried twice by a female (XX) and once by a male (XY), who is hemizygous: he has no second allele to mask a recessive one. The male parent passes his X to every daughter and his Y to every son, so sons receive their X-linked alleles from their mother alone. In birds and butterflies the roles reverse — the male is ZZ and the female ZW — and the tool's ZZ/ZW option swaps them. Offspring phenotypes are reported by sex because the ratio differs between daughters and sons.

Worked Examples

Dihybrid. Mendel crossed plants that were heterozygous for seed shape (round R, wrinkled r) and seed colour (yellow Y, green y). Enter letters R and Y with both parents Rr and Yy. The square has 16 boxes: 9 round yellow, 3 round green, 3 wrinkled yellow and 1 wrinkled green. Mendel's actual F² counts were 315, 108, 101 and 32 out of 556 seeds; against the expected 312.75, 104.25, 104.25 and 34.75, χ² = 0.47 with 3 degrees of freedom, p ≈ .93, so the data fit 9 : 3 : 3 : 1. You can run that test in the chi-square calculator.

Half the sons are white-eyed and no daughter is — the pattern Thomas Hunt Morgan's white-eyed fruit flies revealed in 1910. Enter the gene as W, tick Gene 1 is sex-linked, and choose the carrier female and the dominant male to reproduce it.

Frequently Asked Questions

How do you set up a Punnett square?

Work out each parent's possible gametes — for Aa that is A and a — and write one parent's down the side and the other's across the top. Fill each box with the two alleles that meet there. For two or more genes, combine one allele per gene in every gamete (AaBb makes AB, Ab, aB and ab) so each parent contributes 2ⁿ gametes.

What is the phenotypic ratio of a dihybrid cross?

Two parents heterozygous for two independently assorting genes under complete dominance give 9 : 3 : 3 : 1 — nine showing both dominant traits, three showing only the first, three only the second, and one showing both recessive traits. Other parental genotypes give different ratios; AaBb × aabb, a test cross, gives 1 : 1 : 1 : 1.

How many boxes are in a trihybrid Punnett square?

When both parents are triple heterozygotes each makes eight gametes, so the square has 8 × 8 = 64 boxes, 27 genotypes and 8 phenotypes. Homozygous genes shrink it: a parent who is homozygous for one gene makes only four distinct gametes.

What is a test cross and how do I show it here?

A test cross mates an individual of unknown genotype with a homozygous recessive. Set that parent to aa. If the other parent is Aa, half the offspring are recessive (1 : 1); if it is AA, none are. Seeing the recessive phenotype among the offspring therefore proves the unknown parent carries the recessive allele.

How is a sex-linked Punnett square different?

The sex chromosomes themselves are part of the gamete. A father's X goes only to daughters and his Y only to sons, and males carry one X-linked allele rather than two. That is why X-linked recessive traits such as red–green colour blindness are more common in males, and why the offspring list is split into daughters and sons.

Can this calculator handle linked genes or ABO blood types?

No. It treats genes as unlinked, so it cannot model crossing-over between genes on the same chromosome, and it supports two alleles per gene, so the three-allele ABO system needs a hand-drawn square. Use the codominance setting for two-allele cases such as roan coats.