Mendelian Genetics Calculator
Solve monohybrid and dihybrid Mendelian crosses. Enter parent genotypes using standard allele notation, and instantly see the complete Punnett square with genotype and phenotype ratios. No signup, runs entirely in your browser.
How to Use the Mendelian Genetics Calculator
- 1Enter both parent genotypes using allele letters — uppercase dominant, lowercase recessive.
- 2Use two letters for a monohybrid cross (Aa), four for a dihybrid cross (AaBb). The tool auto-detects which.
- 3Read the completed Punnett square with genotype and phenotype ratios.
- 4Cross an unknown against a homozygous recessive to run a test cross and deduce the hidden genotype.
Worked Example: Two Brown-Eyed Parents, a Blue-Eyed Child
Brown eye colour is (simplifying) dominant over blue. Two brown-eyed parents are surprised by a blue-eyed baby. Enter Bb × Bb: each parent carries one brown allele (B) and one hidden blue allele (b). The Punnett square gives a 1 BB : 2 Bb : 1 bb genotype ratio. Three of the four boxes contain at least one B and show brown eyes; the single bb box shows blue — a 3:1 phenotype ratio, so each child has a 25% chance of blue eyes.
This is the classic resolution of the “how did two brown-eyed parents have a blue child” puzzle: both parents were heterozygous carriers, and the recessive allele was simply masked in them. Notice that 25% is a per-child probability, not a guarantee of one blue child in every four — each conception is an independent draw. Switch the input to BB × Bb and the bb box vanishes entirely (0% blue), which is how a test cross against a known genotype reveals whether a brown-eyed parent is BB or Bb.
Genetics Concepts
Law of segregation
Mendel's first law: each organism carries two alleles per gene, which separate during gamete formation so each gamete carries only one allele. This is reflected in the gamete row and column headers of every Punnett square.
Law of independent assortment
Mendel's second law: genes on different chromosomes assort independently during meiosis. This is why a dihybrid cross produces 4 equally probable gamete combinations. Genes on the same chromosome (linkage) violate this law.
Carrier detection
A carrier is heterozygous (Aa) — they express the dominant phenotype but carry the recessive allele. Two carriers crossing produces a 25% chance of affected offspring (aa). This is the basis of genetic counselling.
Test crosses
A test cross pairs an unknown genotype with a homozygous recessive (aa or aabb). If all offspring show the dominant phenotype, the unknown was homozygous dominant. If half show recessive, it was heterozygous.
Frequently Asked Questions
What is a monohybrid cross?
A monohybrid cross examines the inheritance of a single gene locus with two alleles. For example, Aa × Aa produces offspring in the 1 AA : 2 Aa : 1 aa genotype ratio, giving a 3:1 phenotype ratio (dominant to recessive) when dominance is complete.
What is a dihybrid cross?
A dihybrid cross tracks two independent gene loci simultaneously. AaBb × AaBb uses the FOIL method to produce 4 gamete types each, resulting in a 4×4 grid. The classic 9:3:3:1 phenotype ratio emerges when both loci show independent assortment.
What does uppercase vs lowercase mean?
Uppercase letters represent dominant alleles (e.g. A); lowercase represent recessive alleles (e.g. a). An organism expressing the dominant phenotype can be homozygous dominant (AA) or heterozygous (Aa). Only aa expresses the recessive phenotype.
How do I enter genotypes?
For a monohybrid cross, enter two alleles per parent (e.g. Aa, AA, aa). For a dihybrid cross, enter four alleles (e.g. AaBb). The calculator auto-detects the cross type based on input length.
What is incomplete dominance?
Incomplete dominance occurs when neither allele is fully dominant, producing a blended phenotype in heterozygotes (e.g. red × white flowers → pink). This calculator assumes complete dominance — the dominant allele fully masks the recessive one.
Is the data stored?
No. All calculations are performed in your browser with no data sent to any server.