Pedigree Probability Calculator
Pedigree analysis uses the pattern of a trait across a family tree to work out the probability that a particular individual carries or is affected by a genetic condition. For an autosomal recessive condition, a child is affected only when both parents pass on the recessive allele, which happens when both parents carry it and each transmits it. This calculator estimates the probability that a child is affected by an autosomal recessive condition from the probability that each parent is a carrier. The reasoning follows Mendelian inheritance. If both parents are carriers, each passes the recessive allele to a child with probability one half, so the chance a child receives it from both, and is therefore affected, is one quarter. Multiplying that one quarter by the probability that each parent is actually a carrier gives the overall probability for the child. You enter the carrier probability for each parent, and the calculator returns the probability that a child is affected, expressed as a value between 0 and 1. Every figure is computed deterministically from the probabilities you enter, never guessed, so the same inputs always return the same result. The method and a worked example that reconciles to the calculator default appear in full below.
For an autosomal recessive trait, an affected child needs the recessive allele from both parents. The probability is P1 x P2 x 0.25. With both parents certain carriers (1 and 1), the chance a child is affected is 0.25.
Formula
P(affected child) = P1 x P2 x 1/4
P1, P2 = probability each parent is a carrier
1/4 = chance two carriers have an affected child (Mendelian)
Two carriers of an autosomal recessive allele have an affected child with probability one quarter. Weighting that by the probability that each parent is actually a carrier gives the overall risk for a child.
Worked example
Estimate the risk when both parents are known carriers, so each carrier probability is 1.
- Both parents carriers: 1 x 1 = 1
- Two carriers, affected child probability = 1/4 = 0.25
- Overall = 1 x 0.25 = 0.25
These are the calculator's default inputs, so the affected probability of 0.25 matches the widget exactly.
Pedigree Probability Calculator: frequently asked questions
What is an autosomal recessive condition?
It is a condition that appears only when a person inherits two copies of the recessive allele, one from each parent. Carriers have one copy and are typically unaffected themselves.
Why is the risk one quarter for two carriers?
Each carrier passes the recessive allele with probability one half. The child needs it from both, and one half times one half is one quarter.
How do carrier probabilities enter the calculation?
If a parent is only possibly a carrier, you weight the one-quarter figure by the probability that each parent really carries the allele, multiplying the two carrier probabilities and the one quarter together.
Does this handle dominant conditions?
No. This calculator is for autosomal recessive traits. Dominant and sex-linked conditions follow different probability rules.
Is the result a certainty?
No. It is a probability for any given child. Each pregnancy is an independent event, so the actual outcome can differ from the calculated chance.
Official sources
- Genetics and inheritance reference: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.