Pedigree combined probability calculator

For each mutually exclusive pathway, enter the probability of parent A's genotype, the probability of parent B's genotype, and the conditional transmission probability for the offspring. The calculator multiplies the three independent factors within each pathway, then adds the pathway contributions. It returns every intermediate contribution as well as the combined probability and percentage, making the product and sum rules visible and auditable rather than hiding them behind a single risk figure.

Represent the pedigree question as mutually exclusive pathways

Begin by defining one target outcome, such as an offspring being affected, being a carrier, or inheriting a particular genotype. Then list every parental-genotype combination that can produce that same outcome. Each row is a pathway and needs a unique label, a probability for parent A having the stated genotype, a probability for parent B having the stated genotype, and the probability of the target offspring outcome conditional on that pairing. The pathways must be mutually exclusive: two rows must not describe the same underlying parental state or otherwise overlap. For example, a row for both parents being carriers is distinct from a row for parent A being affected while parent B is a carrier. Do not create two rows that merely rename the same carrier-carrier pairing. Probabilities use decimals from zero through one, so 25% is entered as 0.25. The calculator assumes the two parental genotype probabilities are independent within a row. If they are correlated because of shared ancestry, population structure, or evidence already used for both parents, calculate the appropriate joint parental probability separately before relying on this simplified model.

Apply the product rule within rows and the sum rule across rows

For each pathway, the calculator first multiplies the parent A probability by the parent B probability. This is the joint parental probability under the stated independence assumption. It then multiplies that result by the conditional transmission probability, producing the pathway's offspring-risk contribution. If parent A has a 0.5 carrier probability, parent B has a 0.4 carrier probability, and two carriers have a 0.25 probability of an affected child, the contribution is 0.5 times 0.4 times 0.25, or 0.05. After calculating every row, the tool adds the contributions because the rows represent mutually exclusive alternative routes to the same target outcome. The response includes the joint parental probability, risk contribution, combined decimal probability, and combined percentage, all rounded to twelve decimal places for stable automated use. A combined result above one is rejected because it signals overlapping pathways or inconsistent inputs. Zero is valid and means the supplied pathways contribute no risk. The arithmetic is deterministic and makes no network calls, random draws, or time-dependent adjustments.

Interpret the result without overstating genetic certainty

Treat the combined value as the consequence of the probabilities and assumptions you supplied, not as a diagnosis or a complete clinical risk assessment. Pedigree probabilities may depend on penetrance, phenotype classification, ancestry, de novo variants, test sensitivity, phase, sex linkage, consanguinity, and whether relatives' observations are conditionally independent. This calculator does not infer any of those quantities and does not build a pedigree from family records. It simply combines already established probabilities with the product and sum rules. Check that every transmission value matches the inheritance model and the precise offspring outcome. For an autosomal recessive condition, the affected-child probability for two heterozygous carriers is commonly one quarter, but other parental pairings and other inheritance modes require different conditional values. Review whether the parental probabilities incorporate the same evidence twice, since duplicated evidence can make the independence assumption invalid. Use the visible pathway contributions to audit a worksheet, explain a classroom example, or test software. For medical or reproductive decisions, have a qualified genetics professional review the pedigree, laboratory evidence, inheritance model, and uncertainty. Browser calculations are free; an API request costs $0.002.