How to
How to do a Punnett square
A Punnett square lays out every way two parents' alleles can combine. OpenStax Biology 2e puts it plainly: list one parent's possible gametes along the top, the other's along the side, and fill in the boxes. Each box is one equally likely offspring, so counting boxes gives you the ratios.
A worked example: Yy × Yy
Mendel's pea plants make a good first cross. The allele Y gives yellow seeds and is dominant; y gives green seeds and is recessive. Two terms help here. The genotype is the pair of alleles a plant carries, such as Yy. The phenotype is what you can see, such as yellow. A plant with two identical alleles (YY or yy) is homozygous; one with two different alleles (Yy) is heterozygous.
- Find each parent's gametes. A gamete gets one allele of each pair, so a Yy parent makes Y and y.
- Draw the grid. Put parent 1's gametes across the top and parent 2's down the side. Two gametes each gives a 2 × 2 square.
- Fill each box. Combine the allele at the top of the column with the one at the start of the row: YY, Yy, Yy, yy.
- Count the genotypes. 1 YY, 2 Yy, 1 yy: a 1:2:1 genotype ratio.
- Count the phenotypes. YY and Yy are both yellow, so 3 yellow to 1 green: a 3:1 ratio, or 75% and 25%.
Those are the numbers OpenStax Biology 2e gives for this cross, and they are what the calculator above shows when it opens. Cross YY with yy instead and every box reads Yy: all yellow.
Two or three genes: dihybrid and trihybrid crosses
With two genes, each parent's gametes carry one allele of each. A YyRr plant (yellow, round seeds) makes four kinds: YR, Yr, yR and yr. The square becomes 4 × 4 with 16 boxes, and OpenStax Biology 2e reports the famous 9:3:3:1 phenotype ratio for YyRr × YyRr. Three genes give 8 gametes per parent and 64 boxes; the same textbook gives 27:9:9:9:3:3:3:1 for the trihybrid cross. The calculator draws all 64 boxes and totals them for you, which is where a hand-drawn square usually goes wrong.
To check whether a plant with the dominant trait is YY or Yy, biologists use a test cross: cross it with a homozygous recessive yy. A 1:1 result means the plant was Yy. Try it with the Test cross example.
Incomplete dominance and codominance
Not every allele is fully dominant. In snapdragons, OpenStax Biology 2e describes red (CᴿCᴿ) and white (CᵂCᵂ) parents giving pink (CᴿCᵂ) offspring: incomplete dominance, a blend. Cross two pink plants and you get 1 red, 2 pink, 1 white. In codominance both alleles show fully instead of blending. The human MN blood group is the textbook case: LᴹLᴺ people carry both markers. Either way the ratio is 1:2:1, because the heterozygote now looks different from both homozygotes. Choose the matching option and name the in-between trait.
Blood type (ABO)
ABO has three alleles. According to OpenStax Anatomy and Physiology 2e, Iᴬ and Iᴮ are codominant and i makes a nonfunctional enzyme. So type A can be IᴬIᴬ or Iᴬi, type B can be IᴮIᴮ or Iᴮi, AB is IᴬIᴮ and O is ii. Two parents who are Iᴬi and Iᴮi can have a child of any of the four types, each with a 1 in 4 chance.
Sex-linked (X-linked) traits
Tick On the X chromosome for a gene carried on the X. Parent 1 becomes the mother (two X chromosomes) and parent 2 the father (one X and a Y), and every phenotype shows the sex. For a carrier mother and an unaffected father, OpenStax Anatomy and Physiology 2e explains that no daughter has the condition, each daughter has a 50% chance of being a carrier, and half the sons are affected. Hemophilia and color blindness follow this pattern.
What the calculator assumes
It follows Mendel's rules: each gene sorts into the gametes independently, and every box is equally likely. OpenStax Biology 2e notes that genes sitting close together on the same chromosome tend to be inherited together, so real crosses of linked genes can drift from these ratios. Only one gene can be X-linked, and traits shaped by many genes, such as eye color, are beyond any Punnett square.
Questions people ask
How do you do a Punnett square?
Write one parent's possible gametes across the top of a grid and the other parent's down the side. Fill each box with the alleles from its row and column. Then count the boxes: each one is an equally likely offspring, so the counts give the genotype ratio, and grouping them by trait gives the phenotype ratio.
Are Punnett Squares 100% accurate?
No. A Punnett square gives chances, not a guaranteed result: OpenStax Biology 2e describes it as a way to predict possible outcomes and their expected frequencies. A 3:1 ratio means each offspring has a 75% chance of the dominant trait, yet a litter of four can still come out 2 and 2. It also assumes independent genes.
How to do a Punnett square for blood?
Choose ABO blood type as the inheritance. Type A can be IᴬIᴬ or Iᴬi, type B can be IᴮIᴮ or Iᴮi, type AB is IᴬIᴮ and type O is ii, so pick the genotype for each parent. The square then shows the chance of each child being A, B, AB or O. The Rh factor, positive or negative, is not covered here.
How do I create my own Punnett square?
Pick how many genes you want, then set the allele letters and the trait names for each one, such as Y for yellow and y for green. Choose each parent's genotype and the square, the ratios and the percentages update at once. Download it as PNG or SVG, or copy a link that reopens your exact cross.
How do you make a 4x4 Punnett square?
A 4x4 square comes from a dihybrid cross: two genes, so each parent makes four kinds of gamete. For YyRr the gametes are YR, Yr, yR and yr. Choose Two genes above and the calculator lays out all 16 boxes. When both parents are YyRr, the phenotype ratio comes out 9:3:3:1, the classic result in OpenStax Biology 2e.
Can a Punnett square predict what my baby will look like?
Only for traits controlled by one gene with known alleles, such as ABO blood type. Most visible features depend on many genes. MedlinePlus Genetics, from the US National Library of Medicine, notes that eye color was once taught as brown dominant over blue, a model now known to be too simple because several genes are involved.