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.
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.