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)
Expert Tip
When calculate probabilities for an existing healthy individual, we exclude the "affected" genotype (aa) from the possible outcomes, which changes the divisor (e.g. carrier probability for a sibling becomes 2/3 instead of 2/4).
Methodology & Equations
Conditional Probability (Bayes' Theorem)
For autosomal recessive conditions, if a child is born healthy, they cannot have the genotype aa.
Given raw probabilities P(AA), P(Aa), and P(aa) from the parental cross, the conditional carrier probability is:
P(Aa | healthy) = P(Aa) / [P(AA) + P(Aa)]
This shifts a standard 2/4 carrier probability to 2/3.
Standard Pedigree Values
- **Sibling affected:** Parents must be carriers. Child has 2/3 probability of being a carrier.
- **Parent affected:** Parent is aa. Unaffected child has 100% probability of being a carrier.
- **Aunt/Uncle affected:** Grandparents are carriers. Parent has 2/3 chance. The child's chance is 2/3 × 1/2 = 1/3.
Calculating Pedigree Carrier Risk: Step-by-Step
Here is a step-by-step example where the Mother's sibling is affected (carrier prob 2/3) and the Father has no family history (general population rate 4%):
Determine Parent Genotype Probabilities
- Mother: P(Aa) = 2/3 ≈ 66.7%, P(AA) = 1/3 ≈ 33.3%
- Father: P(Aa) = 4% = 0.04, P(AA) = 96% = 0.96
Sum Genotype Frequencies
- Praw(Aa) = P(M)Aa × P(F)AA × 0.5 + P(M)AA × P(F)Aa × 0.5 + P(M)Aa × P(F)Aa × 0.5
- Praw(Aa) = (2/3 × 0.96 × 0.5) + (1/3 × 0.04 × 0.5) + (2/3 × 0.04 × 0.5) ≈ 0.32 + 0.0067 + 0.0133 = 0.34 (or exactly 34%).
- Praw(aa) = P(M)Aa × P(F)Aa × 0.25 = (2/3 × 0.04 × 0.25) ≈ 0.67%.
- Praw(AA) ≈ 65.33%.
Calculate Conditional Carrier Rate
If the child is healthy, exclude the 0.67% disease risk:
P(Aa | healthy) = 34% / (65.33% + 34%) ≈ 34.2%
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