Estimate whether a person may carry one recessive disease allele. The calculator combines family history, population carrier frequency, negative screening, and partner probability to show residual carrier risk and affected-child risk.
Select a family-history scenario, add carrier screening results, and see risk estimates update without pressing a calculate button.
Load a common autosomal recessive scenario, then edit the prior probabilities or screening detection rate.
Choose a family-history scenario or type a custom prior probability as 1/25, 0.04, or 4%.
Add the reproductive partner probability so the calculator can estimate child risk.
Detection rate means the fraction of true carriers that the test identifies. A negative result leaves residual risk.
Detection
95%
Live autosomal recessive result
That equals about 1 in 150 per pregnancy under a single-gene autosomal recessive model.
Person carrier
66.667%
Partner carrier
4.000%
Affected child
1 in 150
The pedigree view separates family-history probability from partner risk and residual screening risk.
These values combine both posterior carrier probabilities with the Aa × Aa Punnett square.
If both partners carry one recessive allele, every pregnancy has a 25% affected risk, 50% carrier chance, and 25% non-carrier chance.
A carrier probability calculator estimates the chance that an unaffected person carries one altered allele for an autosomal recessive condition. It answers questions like “What is my carrier risk if my sibling is affected?” and “How much does a negative carrier screen reduce my risk?”
Autosomal recessive inheritance requires two altered gene copies for the affected phenotype. A person with one altered copy usually acts as a carrier and can pass that allele to a child. MedlinePlus explains that carriers often show no signs of the condition but can transmit the variant.
This tool uses simple Mendelian probability plus Bayesian updating. It does not name a specific gene, because genes such as CFTR, HBB, SMN1, and HEXA need different carrier frequencies, detection rates, and test panels.
Presets load common classroom cases. They include an unaffected sibling of an affected person, one known carrier parent, two population-frequency partners, and a negative-screen example.
These cards store the prior carrier probabilities before child-risk calculation. You can enter 1/25, 0.04, or 4, and the tool reads each format as a probability.
The switches apply residual-risk logic. A negative result lowers risk according to the detection rate, but it leaves risk from variants outside the test’s detection range.
The result cards combine both partner probabilities. Affected-child risk equals person carrier risk × partner carrier risk × 25% for an autosomal recessive condition.
Select a family-history or population-frequency preset for person 1 and the reproductive partner.
Enter probabilities as fractions such as 1/25, decimals such as 0.04, or percentages such as 4.
Turn on negative screening and set the detection rate when a carrier test failed to detect a variant.
Review posterior carrier probability, affected-child risk, carrier-child chance, and non-carrier chance.
For a known Aa × Aa carrier couple, the Punnett square gives AA, Aa, Aa, and aa. That means 25% non-carrier, 50% carrier, and 25% affected for each pregnancy. MedlinePlus gives the same 25%, 50%, and 25% breakdown for autosomal recessive carrier parents.
For an unaffected sibling of an affected child, the calculator uses conditional probability. The unaffected sibling cannot be aa. Among the remaining AA, Aa, and Aa possibilities, two are carriers. That gives a 2/3 carrier probability.
After a negative carrier screen, the tool applies this residual-risk formula: prior × missed-detection probability divided by total negative-test probability. If the prior is 1/25 and the detection rate is 95%, residual carrier risk becomes about 0.208%, or roughly 1 in 481.
Two carrier parents have an affected child. Another sibling is unaffected. That unaffected sibling has three possible genotypes left: AA, Aa, and Aa.
Two of the three possibilities carry the recessive allele. The sibling’s carrier probability equals 2/3, or 66.67%. If their partner has a 1/25 carrier probability, affected-child risk equals 2/3 × 1/25 × 1/4 = 1/150.
A person starts with a 1/25 carrier estimate. Their carrier screen reports negative, and the panel detects 95% of carriers for the modelled condition.
Residual risk equals 0.04 × 0.05 divided by 0.04 × 0.05 + 0.96. That gives 0.00208, or about 1 in 481. The test lowers risk sharply, but it does not remove all risk.
Carrier probability supports genetic counselling, reproductive planning, and medical genetics education. It helps students connect a pedigree diagram to a numerical risk estimate. It also shows why a partner’s probability matters for autosomal recessive child risk.
Clinical examples include cystic fibrosis involving CFTR, sickle cell disease involving HBB, spinal muscular atrophy involving SMN1, and Tay-Sachs disease involving HEXA. Each condition needs its own carrier frequency and test sensitivity. For that reason, a classroom calculation should not replace a genetics professional’s risk assessment.
You can review autosomal recessive inheritance patterns through MedlinePlus and a broader inheritance overview through OpenStax. MedlinePlus explains autosomal recessive inheritance and OpenStax summarizes carrier inheritance patterns.
This calculator does not diagnose carrier status. It models probability from the values you enter. Real carrier screening depends on gene coverage, variant type, ancestry, family variant information, laboratory method, and residual-risk tables.
The tool also assumes one autosomal recessive locus with complete penetrance. It does not model compound heterozygosity, de novo variants, consanguinity coefficients, pseudodeficiency alleles, copy-number assay limits, or X-linked inheritance.
Use this page for education and preliminary risk practice. Discuss personal carrier screening, pregnancy risk, or family-history interpretation with a qualified genetics professional.
Use these tools to compare autosomal recessive carrier risk with direct Punnett square inheritance and sex-linked transmission.