Punnett Square Calculator

How to Use a Punnett Square

The Punnett square calculator on this page predicts genotype and phenotype ratios for monohybrid and dihybrid crosses. A Punnett square predicts the probability of offspring inheriting specific genotypes — here is how to build one for a monohybrid cross (one trait):

1. Write each parent's genotype (e.g. Aa)
2. List each parent's gametes across the top and side of the grid
3. Fill each cell by combining the gametes from that row and column
4. Count each genotype to find the ratio
5. Determine phenotypes based on which allele is dominant

This calculator does all these steps automatically — just enter the parent genotypes using uppercase letters for dominant alleles and lowercase for recessive.

Monohybrid Cross — The 3:1 Ratio

The most fundamental cross in genetics is Aa × Aa (two heterozygous parents). The offspring ratios are:

• Genotype ratio: 1 AA : 2 Aa : 1 aa
• Phenotype ratio: 3 dominant : 1 recessive
• Probability of dominant phenotype: 75%
• Probability of recessive phenotype: 25%

Mendel observed this ratio in thousands of pea plant crosses, confirming that traits are inherited as discrete units (genes) rather than blending together.

Dihybrid Cross — The 9:3:3:1 Ratio

For a dihybrid cross (AaBb × AaBb), there are 16 possible offspring combinations. The standard expected ratio is:

• 9 — dominant for both traits (A_B_)
• 3 — dominant for trait A, recessive for B (A_bb)
• 3 — recessive for A, dominant for trait B (aaB_)
• 1 — recessive for both traits (aabb)

This 9:3:3:1 ratio holds when: (1) both gene loci are on different chromosomes, or (2) the genes are far enough apart on the same chromosome to assort independently. Genes that are close together on the same chromosome are "linked" and will not follow this ratio.

Common Genetics Problems — Examples

Here are classic Punnett square scenarios:

• Eye color — Brown (B) dominant over blue (b). Bb × Bb → 75% brown, 25% blue
• ABO blood type — Not simple dominance; requires co-dominance (I^A and I^B are co-dominant over i)
• Cystic fibrosis — Autosomal recessive. Carrier × Carrier (Cc × Cc) → 25% affected (cc)
• Pea plant height — Tall (T) dominant over short (t). TT × tt → all Tt (all tall)
• Flower color (incomplete dominance) — Red × White → Pink (not covered by this tool, which assumes complete dominance)

Limitations — When Punnett Squares Don't Apply

Punnett squares assume Mendelian inheritance with complete dominance and independent assortment. They do not account for:

• Incomplete dominance — heterozygotes show intermediate phenotype (e.g. pink flowers)
• Codominance — both alleles expressed simultaneously (e.g. AB blood type)
• Linked genes — genes on the same chromosome that tend to be inherited together
• Multiple alleles — more than two allele forms (e.g. ABO blood type)
• Polygenic traits — traits controlled by many genes (height, skin color)
• Sex-linked traits — genes on X or Y chromosomes (e.g. color blindness)

Genetics and Molecular Biology

While Punnett squares describe inheritance patterns at the organismal level, modern molecular biology analyzes genetic information at the DNA sequence level. For DNA sequence analysis tools, see our Tm calculator which calculates melting temperature and GC content for DNA primers — essential for PCR and genetic analysis techniques. Lab work confirming genetic crosses often requires preparing reagent solutions at precise concentrations; our dilution calculator handles C₁V₁ = C₂V₂ calculations for any buffer or staining solution.