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.
⏱ 7 min read · Complete guide below
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.
The Two Laws That Make It All Work
Every result this calculator produces flows from two principles Gregor Mendel deduced from breeding pea plants in the 1860s. His law of segregation states that each organism carries two alleles for a gene and that these separate during the formation of sex cells, so each gamete receives just one — which is why a heterozygous Aa parent produces A and a gametes in equal numbers along the edge of a Punnett square. His law of independent assortment states that alleles for different genes are distributed into gametes independently of one another, which is why a dihybrid cross yields four equally likely gamete types and the familiar 9:3:3:1 ratio. Remarkably, Mendel worked all this out decades before anyone knew what genes or chromosomes were.
Genotype, Phenotype, and Carriers
The distinction between an organism's genotype (its allele combination, such as Aa) and its phenotype (the trait you can observe) is the heart of Mendelian genetics, and it explains most of its surprises. Because a dominant allele masks a recessive one, the genotypes AA and Aa look identical, so the dominant phenotype can hide a recessive allele in plain sight. An individual carrying a hidden recessive allele is called a carrier, and carriers are why a trait can skip generations and reappear. When two carriers have children, each child has a one-in-four chance of inheriting two recessive alleles and showing the recessive trait — the exact calculation behind genetic counselling for recessive conditions.
Where Simple Mendelian Rules Break Down
This calculator models complete dominance with independently assorting genes, which is the foundation of genetics and covers the classic textbook crosses. Real inheritance is often richer. In incomplete dominance, heterozygotes show a blended phenotype (red and white flowers giving pink); in codominance, both alleles are fully expressed at once (as in AB blood type). Many traits are polygenic, shaped by many genes plus the environment — human height and skin colour are examples that no single Punnett square can predict. And genes located close together on the same chromosome are linked, violating independent assortment. Understanding the simple Mendelian case first is exactly what makes these more complex patterns comprehensible.
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.
What are Mendel's two laws of inheritance?
The law of segregation states that each organism has two alleles per gene, which separate during gamete formation so each gamete carries only one — this is why a heterozygous parent produces two kinds of gamete in equal numbers. The law of independent assortment states that alleles for different genes are distributed into gametes independently, which is why a dihybrid cross gives four equally likely gamete combinations and the 9:3:3:1 ratio. Both underpin every Punnett square this tool builds.
How can two parents without a trait have a child with it?
This happens when both parents are carriers — heterozygous (Aa) for a recessive trait. Each shows the dominant phenotype because the dominant allele masks the recessive one, but each can pass on the hidden recessive allele. When both do, the child is homozygous recessive (aa) and shows the trait. With two carriers, each child has a 25% chance of being affected, which is precisely the situation genetic counselling addresses.
What is a carrier in genetics?
A carrier is an individual who is heterozygous for a recessive allele — they carry one copy of the recessive allele but express the dominant phenotype, so the recessive trait is hidden. Carriers are important because they can pass the recessive allele to their children even though they do not show the trait themselves. This is why recessive conditions can appear to skip generations and then resurface.
Does this calculator handle incomplete dominance or codominance?
No — it assumes complete dominance, where the dominant allele fully masks the recessive one, which is the standard model for introductory Mendelian genetics. Incomplete dominance produces a blended heterozygote (like pink from red and white flowers) and codominance expresses both alleles at once (like AB blood type). Those follow different phenotype ratios and are not modelled here, though the underlying genotype ratios from the Punnett square still apply.
Why do real family ratios not exactly match the predicted ones?
Because a Punnett square gives probabilities, not guarantees. A 3:1 ratio means each child independently has a 75% chance of the dominant phenotype and 25% of the recessive, but any real family is a small sample, so actual outcomes scatter around the prediction — just as flipping a coin a few times rarely gives exactly half heads. The predicted ratios become accurate only across large numbers of offspring.