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A Punnett square is a 2x2 grid that shows every possible gene combination a baby can inherit from two parents. Each parent's two alleles go on the top and side of the grid, then each cell shows one possible combination. Reading the grid gives you the probability of each outcome.
Punnett squares were invented in 1905 by British geneticist Reginald Punnett. They remain the simplest way to calculate inheritance probabilities for single-gene traits.
A Punnett square answers one question: given what alleles each parent carries, what is the probability the baby will inherit each possible combination?
The grid for two heterozygous parents (Bb x Bb) shows:
Since dominant B masks recessive b, the baby will SHOW the dominant trait 75% of the time and the recessive trait 25% of the time.
You need to know what each parent carries. For most simple traits, each parent has two alleles. They might be:
Put one parent's alleles across the top, one set per column. Put the other parent's alleles down the side, one set per row.
Each cell gets the allele from the top of its column combined with the allele from the side of its row.
Count how many cells show each genotype. Each cell represents 25% probability. The combined genotype-to-phenotype map gives you the predicted outcomes.
The MC1R gene controls red hair, and a variant version (r) is recessive. Two parents who are both carriers (Rr x Rr) produce:
So when both parents carry the MC1R red-hair variant, each child has a 25% chance of being a redhead and a 50% chance of carrying it forward.
For the simplified Mendelian model of eye color (this is approximation, the real biology is polygenic), with brown (B) dominant over blue (b):
For a more accurate eye color prediction that accounts for multiple genes, use our baby eye color calculator.
Punnett squares work cleanly only for true single-gene traits. They break down for:
For complex baby appearance traits, AI-based tools like the AI baby face generator at PredictMyBaby work better because they capture the full visible result rather than trying to model gene-by-gene.
You can extend Punnett squares to track two traits at once. With two genes, the grid becomes 4x4 (16 cells). The phenotype ratios from a dihybrid cross of two heterozygotes:
This is the classic 9:3:3:1 ratio Mendel discovered.
In reality, both traits are polygenic, so the actual outcomes are not this clean. But for textbook single-gene examples, the dihybrid cross illustrates how independent assortment produces all possible combinations.
ABO blood type uses three alleles (A, B, O) with codominance. The Punnett square logic still applies. Type A heterozygous (AO) parent x Type B heterozygous (BO) parent produces:
This is one of the few cases where a Punnett square accurately predicts all four blood types from two parents. Codominance makes the result distinct from regular Mendelian dominance.
Parents and genetic counselors use Punnett squares for:
For visualizing actual appearance, including features no Punnett square can predict, parents often use AI baby photo generator tools that work from parent photos rather than allele tracking.
A Punnett square calculates the probability of each genetic combination a baby can inherit from two parents for a single gene. It is used in genetics education, genetic counseling, and predicting outcomes for recessive conditions and simple traits like eye color or blood type.
British geneticist Reginald Punnett invented the Punnett square in 1905 as a teaching tool for explaining Mendel's laws. It became one of the most widely used diagrams in genetics education.
For single-gene traits like blood type or recessive conditions, yes. For most appearance traits like overall face shape, skin tone, or precise eye color, no. Those are polygenic and require more complex tools or AI baby generators that work from parent photos.
For true single-gene Mendelian traits, Punnett squares are mathematically accurate, giving exact probabilities. For traits that are actually polygenic but are taught as Mendelian (like eye color), the results are approximate.
Three-gene Punnett squares require 8x8 grids (64 cells). They become unwieldy quickly. For multi-gene predictions, geneticists use probability calculations or computational tools rather than grids.
You can build a Punnett square for the possible scenarios. If your phenotype is brown eyes, you might be BB or Bb. Build separate squares for each possibility. Genetic testing is the only way to know your exact genotype.
Want a faster way to predict your baby's appearance without doing the Punnett square math? Try the AI baby face generator at PredictMyBaby to see a realistic prediction based on both parents' photos. The AI captures the visible result of all gene interactions, no grid required.