The two kinds of second cousin once removed
Your parent's second cousin. Your mom or dad shares great-grandparents with this person, which makes them second cousins. You're one generation further down, so to you they're a second cousin once removed. They're from your parent's generation, which is why many families just call them "aunt" or "uncle."
Your second cousin's child. You and your second cousin share great-grandparents. Their kid is one generation below you, so to you they're a second cousin once removed. You're their second cousin once removed too: the label works both ways.
Who is my second cousin once removed?
| This person… | …is your |
|---|---|
| Your parent's second cousin | Second cousin once removed |
| Your second cousin's child | Second cousin once removed |
| Your grandparent's second cousin | Second cousin twice removed |
| Your second cousin's grandchild | Second cousin twice removed |
| Your parent's second cousin's child | Third cousin |
| Your first cousin's grandchild | First cousin twice removed |
The quick rule: count how many generations each of you is below the shared ancestor. The smaller number minus one is the cousin number. The difference is how many times removed. Your parent's second cousin is 3 generations below your shared ancestors and you're 4, so: second cousin (3 − 1), once removed (4 − 3). The cousin chart does the counting for you.
How much DNA do you share?
| Relationship | Average cM | Typical range |
|---|---|---|
| Second cousin | 229 | 41–592 |
| Second cousin once removed | 122 | 14–353 |
| Third cousin | 73 | 0–234 |
| Second cousin twice removed | 71 | 0–261 |
Source: Shared cM Project v4. About 1.5% of your DNA, on average. Almost every second cousin once removed shows up as a match on consumer DNA tests, usually labeled "3rd–4th cousin." The ranges overlap a lot, so 120 cM could also be a third cousin or a half second cousin. Ages and family trees settle which. Check any shared cM number.
Is a second cousin once removed closer than a third cousin?
Yes, a little. You share about 122 cM with a second cousin once removed, compared with about 73 cM for a third cousin. Both are distant enough that some pairs share very little DNA.
Can you marry your second cousin once removed?
Yes, in every US state. Even the strictest states only ban marriage between people "nearer of kin than second cousins," and a second cousin once removed is further out than that. The extra genetic risk for children is very small. See cousin marriage laws by state.
For any couple, related or not, the useful question is whether you both carry the same recessive condition. That's what a carrier check answers.