About the Punnett Square Calculator
This Punnett square calculator builds the full grid for a genetic cross and summarises the offspring genotype and phenotype ratios. Enter each parent’s genotype with one letter per gene, uppercase for the dominant allele and lowercase for the recessive — Aa for a monohybrid cross, AaBb for a dihybrid cross, up to four genes — and it lists every gamete, fills every square and counts the results.
It is designed for biology and genetics students working through Mendelian inheritance problems, and for anyone predicting the chance of a trait in offspring. The phenotype ratio uses the A_ notation (A_ means AA or Aa), so a classic AaBb × AaBb cross shows 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb.
The calculation assumes simple Mendelian inheritance: complete dominance (unless you switch on incomplete dominance), autosomal genes and independent assortment with no linkage. Real traits such as eye colour or height are influenced by many genes, so use it for single-gene textbook traits rather than to predict complex human characteristics.
How to use the punnett square calculator
- 1Enter parent 1’s genotype, two letters per gene (Aa, AaBb).
- 2Enter parent 2’s genotype using the same genes in the same order.
- 3Switch on incomplete dominance if heterozygotes look intermediate.
- 4Read the phenotype and genotype ratios and the probability figures.
- 5Use the grid to see which gametes combine into each offspring.
Formula and method
Each parent passes on one allele of every gene. Listing all allele combinations gives that parent’s gametes (Aa → A, a; AaBb → AB, Ab, aB, ab), which form the rows and columns of the square. Every cell combines one gamete from each parent and is equally likely, so probabilities are simply counts divided by the number of cells. Independent assortment is assumed, which is why a dihybrid square has 4 × 4 = 16 cells.
Genotypes are grouped by their exact allele pairs (AA, Aa, aa). With complete dominance, any genotype with at least one uppercase allele shows the dominant phenotype, written A_; only aa shows the recessive. Ratios are reduced by their greatest common divisor, giving familiar results such as 1 : 2 : 1 and 3 : 1 for Aa × Aa.
- A / a
- Dominant / recessive allele of a gene
- A_
- Dominant phenotype (genotype AA or Aa)
Worked examples
Dihybrid cross AaBb × AaBb
Each parent makes four gametes (AB, Ab, aB, ab), giving 16 squares. Nine show both dominant traits, three each show one recessive trait, and one (aabb) shows both — the classic 9 : 3 : 3 : 1 ratio. The all-dominant chance is 9/16 = 56.25%.
Monohybrid cross Aa × Aa
Two carriers each give A or a half the time. The four squares are AA, Aa, aA and aa, so the genotype ratio is 1 : 2 : 1 and three in four offspring show the dominant trait; the chance of an aa child is 25%.
Test cross AaBb × aabb
The aabb parent only makes ab gametes, so each offspring reveals one gamete of the heterozygous parent. The four phenotypes appear in equal 1 : 1 : 1 : 1 proportions if the genes assort independently.
Incomplete dominance: snapdragon colour Rr × Rr
With incomplete dominance the heterozygote is pink rather than red, so the three genotypes give three phenotypes in a 1 red : 2 pink : 1 white ratio.
Frequently asked questions
How do you do a Punnett square?+
Write one parent’s possible gametes across the top and the other’s down the side, then fill each cell with the combination of the row and column alleles. Count the cells of each genotype and phenotype and divide by the total to get ratios and probabilities.
What is the ratio for a dihybrid cross?+
Crossing two double heterozygotes (AaBb × AaBb) gives a 9 : 3 : 3 : 1 phenotype ratio and nine different genotypes in a 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 ratio, assuming complete dominance and independent assortment.
What does the underscore in A_ mean?+
A_ stands for any genotype with at least one dominant allele — AA or Aa — because both show the dominant phenotype. It lets you group squares by what the offspring look like rather than their exact alleles.
What is a test cross?+
A test cross breeds an individual with the dominant phenotype to a homozygous recessive (aa or aabb). If any recessive offspring appear, the tested parent must be heterozygous; a 1 : 1 ratio for one gene confirms it.
Can a Punnett square predict eye colour?+
Only approximately. Eye colour is controlled by several genes, mainly OCA2 and HERC2, so a single-gene Punnett square oversimplifies it. Punnett squares work best for single-gene traits with clear dominance.