Learn & Understand

The Math of Family: What 'Shared DNA' Really Means

Consumer DNA tests report that you share a precise percentage of your DNA with a relative, and family relationships are summarized by a single number called the coefficient of relatedness. Behind these tidy figures lies the arithmetic of inheritance — and a few surprises about how quickly shared DNA thins out across a family tree.

A Number for Closeness

Geneticists express how related two people are as the coefficient of relatedness — the expected fraction of their DNA shared through common descent. A parent and child share half; full siblings share half on average; the figure halves again with each step outward through the family tree. It turns fuzzy words like “second cousin” into an exact expectation.

Why Siblings Aren't Identical

Parent and child share exactly half their DNA, because a child receives precisely half of each parent's genes. But two siblings share half only on average — each inherits an independently shuffled half from each parent, so siblings can share a bit more or less than fifty percent by chance. This randomness is why siblings resemble each other in varying degrees.

The Fast Fade

Shared DNA drops steeply with distance. First cousins share about an eighth; second cousins only a few percent; and beyond that, the shared fraction dwindles fast. Within a handful of generations, distant relatives may share almost no detectable DNA at all — a reason genealogy and DNA testing agree closely for near relatives but grow uncertain for far ones.

Expectation Versus Reality

The tidy coefficients are averages, not guarantees. Because inheritance shuffles DNA randomly at each generation, the actual amount two relatives share fluctuates around the expected value. Consumer tests measure the real, empirical overlap, which is why a reported percentage may differ slightly from the theoretical figure — the dice of recombination, rolled anew in every family.

Looking Up Relatedness

To find the coefficient of relatedness for a relationship, use the Genetic Relatedness Calculator. Measure shared ancestry behind a mating with the Inbreeding Coefficient Calculator, and compare populations with the Genetic Distance Calculator.