Carrier Probability Calculator
Calculate the probability that an unaffected individual is a carrier (Aa) for a recessive genetic disorder based on maternal and paternal family history.
Pedigree & Family History
Select the family history parameters for the mother's and father's sides.
General Population Settings
Default 4% is typical for Cystic Fibrosis in Caucasian populations.
Mother's Family History
Father's Family History
Child Probabilities
If Child is Unaffected / Healthy
General Risk (Before Knowing Health Status)
2/3 Carrier Rule
Unaffected siblings of affected individuals have a 2/3 (66.7%) chance of being carriers because the affected aa genotype is excluded.
Pedigree Relationship Carrier Risk Reference Table
| Relationship to Affected Individual | Parent Genotype Assumptions | Prior Probability | Carrier Risk (Aa) |
|---|---|---|---|
| Child of Affected Parent (aa) | aa × AA/Aa | 100% | 100% (Obligate Carrier) |
| Unaffected Sibling of Affected Person | Aa × Aa | 2/4 | 2/3 (66.7%) |
| Niece / Nephew of Affected Person | Aa (2/3) × Pop | 2/3 × 1/2 | 1/3 (33.3%) |
| First Cousin of Affected Person | Aa (1/3) × Pop | 1/3 × 1/2 | 1/6 (16.7%) |
Calculation Methodology & Biological Principles
Conditional Probability Formula
For healthy individuals, we exclude the recessive disease outcome (aa):
Hardy-Weinberg Integration
When a spouse has no known family history of a disorder, their prior carrier probability is estimated using the general population carrier frequency (2pq).
Frequently Asked Questions
What is a Genetic Carrier? ▼
A carrier is an individual who inherits one recessive allele for a genetic disorder (Aa) without displaying symptoms. They can pass the recessive allele to their children.
Why is an unaffected sibling's carrier probability 2/3 instead of 1/2? ▼
If a sibling has an autosomal recessive disorder (aa), both parents must be carriers (Aa x Aa). A standard cross yields 1 AA : 2 Aa : 1 aa. If the individual is known to be healthy, the affected aa outcome is ruled out, leaving 2 carriers out of 3 healthy outcomes (2/3 or ~66.7%).
How does Bayes' Theorem apply to carrier risk assessment? ▼
Bayes' theorem incorporates clinical observations (such as an individual or ancestor being unaffected) to update prior genetic probabilities into conditional posterior probabilities.
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