Compute per-child and cumulative autosomal recessive disease risk from parental statuses (carrier / affected / non-carrier / unknown via population carrier rate), including the 2/3 carrier rule. Derived from Griffiths, Hartl & Clark 2007, StatPearls. Educational use only.
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Key facts
Category
Education
Input types
select, number
Output type
json
Sample coverage
4
API ready
Yes
Overview
The Autosomal Recessive Inheritance Risk Calculator estimates the affected-child risk for two parental genotype statuses and the cumulative chance that at least one of a specified number of children is affected. It supports carrier, affected, not-a-carrier, and unknown statuses, using a population carrier rate for unknown parents. Educational use only.
When to use
Estimate the per-child risk for an autosomal recessive condition from known parental statuses.
Use a population carrier rate when one parent's genotype status is unknown.
Calculate the cumulative probability that at least one of multiple children is affected.
How it works
1Select a status for each parent: carrier (Aa), affected (aa), not a carrier (AA), or unknown.
2Enter the population carrier rate when a parent's status is unknown, along with the number of children.
3The calculator combines each parent's probability of transmitting the recessive allele to determine the per-child affected risk.
4It reports the per-child result and the cumulative risk for at least one affected child using 1 − (1−r)^n; for two carriers, it also applies the 2/3 carrier probability among unaffected children.
Use cases
Teaching Mendelian inheritance with carrier, affected, and non-carrier genotype combinations.
Exploring how an unknown partner's population carrier rate changes recurrence risk.
Comparing single-child risk with the chance that at least one child is affected across several children.
Examples
1. Two carrier parents: 25% per child
Genetics student
Background
A student is reviewing autosomal recessive inheritance using two confirmed carriers, represented as Aa × Aa.
Problem
Calculate the affected, carrier, and non-carrier probabilities for each child and the risk across two children.
How to use
Select Carrier (Aa) for both parents, keep the population carrier rate at 0.04, enter 2 children, and choose 4 decimal places.
Parent 1: Carrier (Aa); Parent 2: Carrier (Aa); Population carrier rate: 0.04; Number of children: 2; Decimal places: 4.
Outcome
The per-child affected risk is 25%, the carrier probability is 50%, and the probability that at least one of two children is affected is 43.75%. An unaffected child has a 2/3 carrier probability.
2. Carrier and unknown partner: 1.08% per child
Genetics educator
Background
An educator wants to show how an unknown partner's population carrier rate changes the risk when the other parent is a confirmed carrier.
Problem
Estimate the affected-child risk when one parent is Aa and the unknown parent's population carrier rate is 4%.
FAQ
What parental statuses does the calculator support?
It supports carrier (Aa), affected (aa), not a carrier (AA), and unknown.
What does the population carrier rate mean?
It is the value c used to estimate an unknown parent's status, with P(Aa)=c and P(aa)=c².
What is the risk when both parents are carriers?
Each child has a 25% affected risk, a 50% carrier probability, and a 25% probability of being a non-carrier.
What is the 2/3 carrier rule?
For two carrier parents, an unaffected child has a 2/3 probability of being a carrier.
Is this calculator a substitute for genetic counseling?
No. It is intended for education and does not replace professional genetic counseling.