What is a Punnett square?
The Punnett square is a visual tool created by the English geneticist Reginald Punnett in 1905 to predict the genotypes and phenotypes of offspring from the cross of two individuals. It is one of the pillars of the study of Mendel’s Laws.
How it works
Each parent contributes a gamete (sperm or egg), which carries one allele of each gene. In the square:
- The columns represent the possible gametes of one parent
- The rows represent the gametes of the other
- Each cell shows a possible genotype of the child
- The proportion of cells indicates the probability of each combination
Genetic notation
- Uppercase letter (A) = dominant allele
- Lowercase letter (a) = recessive allele
- AA = homozygous dominant (pure)
- aa = homozygous recessive
- Aa = heterozygous (hybrid)
Classic example: Aa × Aa
The cross between two heterozygotes produces the Mendelian 3:1 phenotypic ratio (3 dominant : 1 recessive) and the 1:2:1 genotypic ratio (1 AA : 2 Aa : 1 aa). This was Mendel’s fundamental observation when crossing pea plants.
Dihybrid crosses
In crosses with two genes (e.g. AaBb × AaBb), the square becomes 4×4 = 16 combinations, producing the famous 9:3:3:1 ratio. This confirms Mendel’s 2nd Law (independent assortment of genes).
Frequently asked questions
What does a 3:1 result in a Punnett square mean?
In Aa × Aa, 3 out of every 4 offspring show the dominant phenotype and 1 out of 4 shows the recessive one. It is the classic phenotypic ratio of Mendel’s 1st Law.
Can I use it for more than 2 genes?
This calculator supports up to 2 gene pairs (dihybrid crosses). For 3 or more, the square becomes three-dimensional and it is easier to calculate the probabilities by multiplying the ratios of each gene separately.
Does the calculator account for incomplete dominance or codominance?
No. It works with simple dominance (an uppercase letter dominates a lowercase one). In cases like snapdragon flower color (incomplete dominance) or blood type (multiple alleles), the phenotypic ratios are different.
What are alleles?
They are variants of the same gene. For example, the flower color gene may have an R allele (red) and an r allele (white). Each individual inherits 2 alleles: one from the father and one from the mother.
Why use gametes instead of genotypes directly?
Because in meiosis the allele pairs separate — each gamete carries only one allele of each gene (Law of Segregation). That separation is what the Punnett square models.
What is a test cross?
It is the cross between an individual with a dominant phenotype (unknown genotype, AA or Aa) and a homozygous recessive (aa). By looking at the offspring, you can tell whether the first one is homozygous or heterozygous.