Pedigree Analysis Calculator
Build a family pedigree and let the calculator work out which modes of inheritance are still possible and which are logically ruled out, calculate offspring risk once you know the mode, or find out how much extra risk a related couple actually faces — with a drawn pedigree diagram and full step-by-step reasoning.
Background
A pedigree is a diagram of a family's phenotypes across generations — squares for males, circles for females, filled in for "affected." The pattern of who is and isn't affected, and how that pattern moves through a family, is enough to rule out most modes of inheritance using nothing but simple, airtight logic.
How to use this calculator
- Choose Identify the Pattern to build a small pedigree and see which inheritance modes are logically consistent with it.
- Choose Offspring Risk once you already know the inheritance mode and want the probability of an affected, carrier, or unaffected child.
- Choose Consanguinity Risk to see how much extra risk related parents (like first cousins) add for a recessive condition.
- Click Calculate to see the pedigree diagram (or Punnett-style grid) plus a full step-by-step explanation.
How pedigree logic works
If two unaffected parents have an affected child, the trait can't be dominant — a dominant allele has to come from somewhere. This rules out both autosomal and X-linked dominant.
If two affected parents (aa × aa) have an unaffected child, the trait can't be recessive — aa × aa can only ever produce aa. This rules out both autosomal and X-linked recessive.
An affected X-linked recessive mother (XaXa) passes her X to every son, so all her sons must be affected — an unaffected son rules out X-linked recessive.
An affected X-linked dominant father (XDY) passes his X to every daughter, so all his daughters must be affected — an unaffected daughter rules out X-linked dominant.
A small pedigree usually can't prove a single mode beyond doubt — it can only rule impossible ones out. Whatever's left after ruling out is the best-supported answer.
Formula & Equations Used
Autosomal cross: standard single-gene Punnett square over the two parents' alleles.
X-linked cross: daughters get one X from each parent; sons get their father's Y and their mother's X — computed separately by sex.
Coefficient of relationship (r): the probability two relatives share a given allele by common descent — 1/2 for siblings, 1/8 for first cousins, and so on.
Inbreeding coefficient of offspring: F = r ÷ 2
Genotype frequency under inbreeding (Wright's formula): freq(aa) = q² + F·q·(1−q)
Example Problems & Step-by-Step Solutions
Example 1 — Ruling out dominant inheritance
Two unaffected parents have an affected daughter.
Step: a dominant trait requires at least one affected parent (assuming no new mutation) — since neither parent is affected, autosomal dominant and X-linked dominant are both ruled out.
Result: the trait is recessive — either autosomal or X-linked, more data needed to tell which.
Example 2 — Distinguishing X-linked recessive
An affected mother has an unaffected son.
Step: an X-linked recessive mother (XaXa) passes Xa to every son — an unaffected son is impossible under that model.
Result: X-linked recessive is ruled out — the trait must be autosomal recessive.
Example 3 — Offspring risk, two carriers
Both parents are Aa carriers for an autosomal recessive condition.
Step: Aa × Aa → 1/4 AA, 1/2 Aa, 1/4 aa.
Result: 25% chance of an affected (aa) child, 50% unaffected carrier, 25% unaffected non-carrier.
Example 4 — Consanguinity risk
First cousins (r = 1/8) have a child; the recessive allele frequency is q = 0.01.
Step: F = 1/8 ÷ 2 = 0.0625. Risk = 0.01² + 0.0625×0.01×0.99 ≈ 0.00072.
Result: about 1 in 1,390 — roughly 7× the baseline risk for unrelated parents (1 in 10,000).
Frequently Asked Questions
Can a pedigree ever prove an inheritance mode with certainty?
Rarely from a small family alone. Pedigree analysis works mainly by ruling modes out — a small pedigree can easily be consistent with more than one mode, and larger families or additional data (like population allele frequency) are often needed to fully distinguish the rest.
What assumptions does this calculator make?
It assumes a single gene with complete penetrance, no new (de novo) mutations, and no genetic phenomena like incomplete penetrance, epistasis, or mitochondrial inheritance. Real pedigrees can be messier than this — this tool covers the standard classroom case.
Why does an affected father never pass an X-linked trait to his sons?
Sons get their Y chromosome from their father and their single X chromosome from their mother — so whatever's on the father's X chromosome simply never reaches a son. Any X-linked trait a son has came from his mother.
Why do related parents raise the risk of a recessive condition?
Related people are more likely to carry the very same rare allele, inherited from a shared ancestor, than two random unrelated people are. That shared ancestry is exactly what the coefficient of relationship measures, and it's why related couples have a higher chance of both being carriers of the same recessive allele.
Is "unaffected" the same as "not a carrier"?
No — for a recessive trait, an unaffected person can still be a heterozygous carrier (Aa). This calculator's Offspring Risk mode asks for exact genotypes rather than just phenotypes, specifically to avoid that ambiguity.
Why is X-linked dominant inheritance relatively rare in pedigrees?
It's not that it's biologically rarer — it's that far fewer conditions happen to be caused by dominant X-linked genes. When it does occur, its clearest tell is that an affected father passes the trait to all of his daughters and none of his sons.
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