Relationship guides
One page per relationship, written to actually settle the question
Each guide covers the definition, the path through the tree, the expected DNA, the real range, the relationships it overlaps with, and what evidence breaks the tie.
First cousin
Typically 829 cM · 266–1,516 cM
First cousins share grandparents and, on average, about an eighth of their DNA. Here is the range to actually expect, and the relationships that sit on top of it.
Second cousin
Typically 194 cM · 7–677 cM
Second cousins share great-grandparents and around 3% of their DNA — but the realistic range runs from under 50 cM to over 500. This is where a single number stops being enough.
Half sibling
Typically 1,718 cM · 908–2,546 cM
Half siblings share one parent and about a quarter of their DNA — the same expected amount as a grandparent, an aunt or a niece. It is the most consequential ambiguity in genetic genealogy.
More relationships are on the way
Rather than generating dozens of thin pages by swapping numbers, each relationship guide is written once, properly, with its own tree diagram, range chart and confusion set. In the meantime, every relationship in the dataset is covered by the calculators.
Look up any relationshipMore relationship guides
Aunt or uncle DNA: what the match can and cannot tell you
How much DNA an aunt, uncle, niece, or nephew can share and how to distinguish that path from a half sibling or grandparent.
Grandparent DNA: average sharing and real-world checks
What a grandparent or grandchild DNA match usually looks like and why it can overlap with other close relationships.
First cousin once removed: two directions, one name
A plain-English explanation of first cousin once removed, how to draw the family connection, and what DNA overlap to expect.
Third cousins: related on paper, variable in DNA
How third cousins connect, why their DNA totals vary widely, and why a real third cousin may not match at all.
Half aunt or half uncle: one shared grandparent
What makes an aunt or uncle “half,” how that changes the family tree, and why their DNA can resemble other close relationships.
Great aunt or great uncle: one generation above your aunt or uncle
How great aunts and uncles fit in a family tree, what DNA sharing to expect, and why they can be confused with first cousins.
Half first cousins: one shared grandparent, not two
A clear explanation of half first cousins, the family path behind the name, and how to distinguish them from full first cousins.
Double first cousins: two cousin paths at once
Why double first cousins share more DNA than ordinary first cousins and which close relationships their totals can resemble.
Three-quarter siblings: a rare relationship with a real DNA pattern
What three-quarter siblings are, why they share more DNA than half siblings, and how to handle the question with care.
Great-grandparent DNA: expected share versus what you actually inherit
How much autosomal DNA may come from a great-grandparent, why the amount varies, and why family descent is larger than a percentage.
Second cousin once removed: the generational difference matters
How second cousin once removed works in both directions, how it differs from second and third cousin, and why DNA overlap is expected.
Half sibling vs first cousin: what DNA can separate
Why half siblings and first cousins may overlap in shared cM, and which DNA and family evidence has the power to distinguish them.
Aunt vs half sibling: a close-match question DNA cannot answer alone
How to approach a DNA match that could be an aunt, uncle, niece, nephew, or half sibling without overstating what a cM total proves.