Mendelian Ratio Calculator
Predict wild-type, het, and homozygous offspring ratios for knockout and transgenic crosses, and see how many pups you need to genotype to hit your target.
Try Moustra freeHow Mendelian ratios work
Gregor Mendel's law of segregation states that each parent passes one of its two alleles to each offspring at random. For a single gene with a wild-type allele (+) and a mutant allele (m), crossing two heterozygous parents (+/m × +/m) yields offspring in the classic 1 : 2 : 1 ratio — 25 % wild-type (+/+), 50 % heterozygous (+/m), and 25 % homozygous mutant (m/m).
When you are working with multiple independent transgenes or knockout alleles, Mendel's law of independent assortment applies: genes on separate chromosomes are inherited independently, so the probability of any multi-gene combination is simply the product of the individual per-gene probabilities. A double homozygous knockout from two het × het crosses, for example, is expected in 1/4 × 1/4 = 1/16 (≈ 6.25 %) of offspring.
These ratios are population averages. Any individual litter can deviate substantially — especially with small litter sizes. Use the pups-to-genotype estimate to plan colony crosses that give you a statistically high chance of obtaining at least one animal with the target genotype.
How to use this calculator
- Add one row per gene — click "Add gene" for each additional locus (up to 5). Give each gene a meaningful name so the results table is easy to read.
- Set parent zygosities — choose wild-type (+/+), heterozygous (+/m), or homozygous (m/m) for each parent. The Punnett square is calculated instantly.
- Choose the desired offspring zygosity — select the genotype you are trying to obtain for each gene. The calculator multiplies the per-gene probabilities to give the combined probability.
- Enter your sample size — type how many pups you plan to genotype, or typically obtain per litter, to see how many matches to expect.
- Set your confidence level — choose 90 %, 95 %, or 99 % to see how many pups you need to genotype to have that probability of obtaining at least one animal with the full target genotype.
- Share or export — copy the URL to share your exact cross setup, or download a CSV for your lab notebook.
Frequently Asked Questions
What is a Mendelian ratio in mouse genetics?
A Mendelian ratio describes the expected proportions of offspring genotypes from a cross between two parents, following Mendel's laws of segregation. For a single gene, a heterozygous × heterozygous cross (+/m × +/m) produces wild-type, heterozygous, and homozygous offspring in a 1 : 2 : 1 ratio, meaning 25 % of pups should be homozygous knockouts.
Why does a het × het cross give a 1 : 2 : 1 ratio?
Each heterozygous parent carries one wild-type (+) and one mutant (m) allele. A Punnett square gives four equally likely combinations: +/+ (wild-type), +/m (het), +/m (het), and m/m (homozygous). That is 1 wild-type : 2 heterozygous : 1 homozygous, or 25 % : 50 % : 25 % of offspring.
How do I calculate expected ratios for multi-gene crosses?
When genes assort independently (i.e., they are on different chromosomes or far apart on the same chromosome), the combined probability of a specific multi-gene genotype is the product of the individual per-gene probabilities. For example, a double homozygous knockout from two independent het × het crosses has a probability of 0.25 × 0.25 = 0.0625, or 1 in 16. This calculator multiplies across all genes you add.
Why do my actual genotyping counts deviate from the expected ratio?
Mendelian ratios are population-level expectations, not guarantees for any single litter. With small sample sizes, random chance causes large deviations. A litter of 8 pups from a het × het cross might yield 0 or even 4 homozygous pups by chance alone. The expected-matches and pups-to-genotype numbers help you plan cohort sizes so you have a high probability of obtaining at least one animal of the desired genotype.
How many pups do I need to genotype to get a double knockout?
For a double homozygous knockout from two independent het × het crosses the probability per pup is 1/16 (≈ 6.25 %). To have a 95 % chance of obtaining at least one double knockout you need to genotype approximately 46 pups (the calculator shows this instantly). The exact number scales with the number of genes and the parents' zygosities.
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