Every parent passes about half of their DNA to a child. Counting those halvings gives the textbook figure for each relationship: the expected share. A real test gives a figure that moves around it.
The arithmetic
For relatives who share a couple as common ancestors, the expected shared DNA is 100% times one half to the power of (a + b − 1), where a and b are the generations to the common ancestor. So a grandparent shares 25%, an aunt or uncle 25%, a first cousin 12.5%, a second cousin 3.125% and a third cousin about 0.78%. For half relatives the figure halves, because only one ancestor is shared. A full sibling, a special case, is expected to share around half.
Why real results vary
DNA is inherited in whole chunks, not in neat halves. Two first cousins might share noticeably more or less than 12.5%. By the third and fourth cousin level, some real cousins share no detectable DNA at all, even though they are related. Testing companies therefore report ranges, and a match at a given amount is compatible with several different relationships.
Reading the number
- Use Kinage's percentage as an average, never as a verdict.
- Adoptive, step and in-law links carry no expected biological DNA, and Kinage says so.
- A single large match is better evidence than a long list of small ones.
Every term page gives the average for its relationship, and the full and half guide explains why the figure changes.