Pedigree Probability Calculator

Background.

Your brother was born with an autosomal recessive condition and you were not. Both your parents must be carriers, so what is the chance that you carry it too? Most people say one half. The answer given in genetic counselling is two thirds, and the gap comes from using what is already known. At your conception there were four equally likely outcomes — affected, carrier from your mother, carrier from your father, or neither variant. You are unaffected, which removes the first, and two of the remaining three make you a carrier.

That figure is the prior. The next step weighs evidence that has arrived since. If you and a partner who carries already have unaffected children, each of them was a little less likely if you carry (a 3 in 4 chance per child) than if you do not (certain), so your carrier probability moves down. It moves less than intuition expects: three unaffected children take two thirds only to about 46%, and no run of healthy children ever brings it to zero.

Three pedigree positions are covered: the unaffected sibling of someone with an autosomal recessive condition, the child of one known carrier, and the sister of a man with an X-linked recessive condition whose mother is a carrier. For each you see the prior, the likelihood ratio of the evidence, the updated carrier probability and the forward risks for the next child.

These are the textbook Mendelian calculations taught in genetics courses. They leave out carrier test results, which in real families revise the risk far more than any family history, and they are not a clinical risk assessment.

What is pedigree probability calculator?

Pedigree probability is Bayes' theorem applied to two hypotheses: this person carries the variant, or does not. The prior comes from where they sit in the family tree. For each hypothesis you then ask how likely the observed evidence — here, the unaffected children already born — would have been. Multiplying prior by that conditional probability gives a joint probability, and dividing each joint by the sum of both gives the posterior. Genetic counsellors set this out as a four-row table of prior, conditional, joint and posterior. Its strength is that every step is explicit, so the effect of each new observation can be seen and checked.

How to use this calculator.

1. 01Choose where the person sits in the pedigree; this sets the prior at 2/3 for an unaffected sibling of an affected person, 1/2 for the child of one carrier, and 1/2 for the sister of an affected man in an X-linked family.
2. 02Enter the unaffected sons and daughters that person already has, as whole numbers up to 100 each, or 0 if there are none. With no children the posterior simply equals the prior.
3. 03Set the partner's carrier probability: 100% for a partner known to carry, which is what most textbook problems intend; otherwise the figure from a laboratory report or professional guideline. The X-linked scenario ignores it and assumes an unaffected father.
4. 04Read the prior, the likelihood ratio and the posterior together, then the next-child risks for the forward question.
5. 05Take any real decision, especially in pregnancy, to a clinical geneticist or certified genetic counsellor, who can add test results the model cannot.

The formula.

The posterior is prior × P(E|C) ÷ [prior × P(E|C) + (1 − prior) × P(E|not C)], where E is the set of unaffected children and C means the person carries. In this model a non-carrier cannot have an affected child, so P(E|not C) = 1 and the likelihood ratio equals P(E|C), which can never exceed 1: healthy children can count against carrying but never for it. In the autosomal scenarios, with k unaffected children of either sex and a partner who carries with probability q, P(E|C) = q × (3/4)^k + (1 − q). The first term covers a carrier partner, where each child escapes with probability 3/4; the second a non-carrier partner, where no child could have been affected anyway. For the next child, the four joint states — both parents carry, only this person, only the partner, neither — are each updated, then risk affected = P(both carry | E) × 1/4 and risk of an unaffected carrier = 1/2 × [1 − P(neither carries | E)]. In the X-linked scenario only sons inform, P(E|C) = (1/2)^s for s unaffected sons; the next son is affected with probability posterior × 1/2, any next child with posterior × 1/4, and a daughter with 0, because her father passes a working X. The model assumes full penetrance, a single gene, no new mutation or germline mosaicism and an accurate family history; reduced penetrance in particular would weaken each unaffected child as evidence and leave the true posterior higher than shown.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Moskowitz S. M., Chmiel J. F., Sternen D. L., et al. (2008). Clinical practice and genetic counseling for cystic fibrosis and CFTR-related disorders. Genetics in Medicine 10(12):851–868. doi:10.1097/GIM.0b013e31818e55a2, PMID 19092437. Source of the 2/3 prior, verbatim: 'For an at-risk sibling who is known to be unaffected but has not yet undergone molecular genetic testing, the risk of being a carrier is 2/3.' Also the source for the point that residual risk after a negative carrier test depends on the carrier frequency and the mutation-detection rate. Open access via PMC. Retrieved 2026-07-29.
2. [2]Savant A., Lyman B., Bojanowski C. & Upadia J. Cystic Fibrosis. GeneReviews®, University of Washington, Seattle; NCBI Bookshelf ID NBK1250, last revision 8 August 2024. Independent source of the at-conception figures from which the 2/3 prior follows: 'each sib of an affected individual has at conception a 25% chance of being affected, a 50% chance of being heterozygous, and a 25% chance of inheriting neither of the familial pathogenic variants.' Conditioning on being unaffected gives 0.50/(0.50+0.25) = 2/3. Free full text. Retrieved 2026-07-29.
3. [3]Gulani A. & Weiler T. Genetics, Autosomal Recessive. StatPearls, NCBI Bookshelf ID NBK546620, last update 1 May 2023. Source for the segregation figures underlying the autosomal likelihoods — 'Each parent has a 50% chance of passing on the disease allele' and the 25% affected / 50% carrier / 25% homozygous dominant outcome of a carrier × carrier cross. Free full text. Retrieved 2026-07-29.
4. [4]Bin Shlhoob R., Tanaka J. & Pandya A. M. Genetics, X-Linked Inheritance. StatPearls, NCBI Bookshelf ID NBK557383, last update 22 March 2026. Source of the X-linked prior and likelihood: for a carrier mother, 'each male offspring has a 50% chance of being affected, and each female offspring has a 50% chance of being a carrier.' Free full text. Retrieved 2026-07-29.
5. [5]Ogino S. & Wilson R. B. (2004). Bayesian analysis and risk assessment in genetic counseling and testing. Journal of Molecular Diagnostics 6(1):1–9. PMID 14736820. The standard methods reference for the prior / conditional / joint / posterior table structure implemented here. PROVENANCE NOTE: bibliographic reference only — the publisher's full text was not retrievable and no free copy was located, so no numeric figure on this page is attributed to it. Cited by PMID so the record is identifiable rather than presented as a working link.
6. [6]MedlinePlus Genetics, US National Library of Medicine. Inheritance Patterns, page last updated 19 April 2021. Source for the inheritance-mode definitions framing the three pedigree scenarios, including that the parents of an individual with an autosomal recessive condition 'each carry one copy of the altered gene, but they typically do not show signs and symptoms.' Free. Retrieved 2026-07-29.