Genetic Relatedness Calculator
Quantifying "How Related" Two Relatives Are
Family relationship terms like "second cousin" or "half sibling" describe a pedigree structure, but geneticists express the closeness of that relationship as a single number: the coefficient of relatedness, r — the expected proportion of genetic material two individuals share purely by common descent. This reference tool looks up the standard, well-established r value for common relationship types, a figure used throughout population and behavioral genetics.
The Reference Values
These coefficients come from standard pedigree analysis (Wright's method of path counting) and are treated as established constants in genetics, not something this calculator computes from scratch:
| Relationship | r | Approx. Shared DNA |
|---|---|---|
| Identical twins / clone | 1.0 | 100% |
| Parent – Child | 0.5 | 50% |
| Full siblings | 0.5 | 50% |
| Half siblings | 0.25 | 25% |
| Grandparent – Grandchild | 0.25 | 25% |
| Aunt/Uncle – Niece/Nephew | 0.25 | 25% |
| Great-Grandparent – Great-Grandchild | 0.125 | 12.5% |
| First cousins | 0.125 | 12.5% |
| Half first cousins | 0.0625 | 6.25% |
| Second cousins | 0.03125 | 3.125% |
These are theoretical expectations based on pedigree structure and random Mendelian assortment. Real, empirically measured shared-DNA percentages (as reported by consumer DNA testing services) can vary somewhat around these figures due to the randomness of actual recombination.
Where This Reference Is Used
- Population and behavioral genetics coursework — relatedness coefficients underpin core concepts like kin selection and Hamilton's rule.
- Interpreting consumer DNA test results — comparing a reported shared-DNA percentage against the theoretical expectation for a claimed relationship.
- Animal breeding and pedigree management — relatedness coefficients between breeding stock inform decisions aimed at maintaining genetic diversity in a herd or breeding line.
How to Use This Calculator
- Select the relationship type from the list.
- Select Calculate to see the standard coefficient of relatedness (r) and its approximate shared-DNA percentage.
Related Calculations
For the inbreeding coefficient of offspring from related parents, see the Inbreeding Coefficient Calculator, or explore population-level allele math with the Allele Frequency Calculator.