First cousins share about 12.5% of their dna on average, with real results ranging from roughly 7% to 14%.
Understanding What First Cousin Dna Sharing Means
When you ask how much dna do you share with a first cousin?, you are really asking how closely the two of you sit on the family tree. A first cousin is the child of your aunt or uncle, so you share a set of grandparents. Those shared grandparents pass dna down to your parent and to your cousin’s parent, and then again down to you and your cousin.
Geneticists describe this connection using the coefficient of relationship. For first cousins that number is 0.125, which translates to 12.5% shared dna between two relatives on average. That figure lines up with classic population genetics work and with large data sets collected from modern consumer tests.
How Much Dna Do You Share With A First Cousin By Percentage?
On paper, the textbook answer to how much dna do you share with a first cousin? is 12.5% shared dna. That number comes from simple probability rules. Each parent passes half of their dna to a child, and you and your cousin each receive half of your shared grandparent’s dna through your different parents.
Real dna tests show a range rather than a single number. Data from large testing companies shows first cousins usually share between a little over 7% and nearly 14% of their autosomal dna, with many pairs clustering close to the 12.5% mark. That spread reflects the random way chromosomes reshuffle when eggs and sperm form.
Average Shared Dna By Relationship
It helps to see first cousin sharing in context with other close relatives. The table below summarises average dna sharing and typical centimorgan ranges across a few common relationships. A centimorgan, often shortened to cM, is a unit that measures how much dna two people share along their chromosomes.
| Relationship | Average Dna Shared | Typical Range (cM) |
|---|---|---|
| Parent / child | 50% | Around 3400 cM |
| Full siblings | 50% | Roughly 2200–3020 cM |
| Half siblings | 25% | About 1340–2150 cM |
| Grandparent / grandchild | 25% | Around 1700–2100 cM |
| Aunt / uncle with niece / nephew | 25% | About 1300–2300 cM |
| First cousins | 12.5% | Roughly 396–1397 cM |
| Second cousins | 3.125% | About 46–515 cM |
These values come from aggregated test data and from classic genetic models that describe how dna recombines in each generation. Public resources such as the percent dna shared by relationship table explain how these averages and ranges are calculated and how they apply to real test reports.
Why First Cousins Share About One Eighth Of Their Dna
The 12.5% figure rests on basic inheritance rules. You receive half of your dna from each parent. Each parent in turn received half of their dna from each grandparent. When you trace the path from a shared grandparent down through your parent and then to you, the fraction of that grandparent’s dna in your genome is one quarter. Your cousin’s share from the same grandparent is also one quarter, so the expected overlap between the two of you is one eighth of your total dna.
The genome does not split into neat mathematical blocks, though. During the formation of eggs and sperm, chromosomes swap pieces through recombination events. This shuffle creates fresh dna segment combinations in every generation and pushes real cousin pairs slightly above or below the textbook 12.5% value.
Centimorgans And Dna Segments Between First Cousins
Dna tests report shared dna in centimorgans rather than plain percentages. For first cousins, the average shared amount sits around 866 cM. Large projects that pool test results show a common range from roughly 396 cM at the low end to nearly 1400 cM at the high end. Community tools such as the shared cM project chart turn those numbers into relationship probabilities that align well with what users see in their match lists.
The total cM is only one part of the picture. Tests also show how many segments you share and how long those segments are. First cousins often share several long segments that reflect the recent shared ancestry through the grandparents. More distant cousins share fewer and shorter segments because recombination has broken the original blocks into smaller pieces over extra generations.
Comparing First Cousins With Other Cousin Types
Many testers wonder why a match sits near the first cousin range but not quite inside it. That is where comparison with half first cousins, cousins once removed, and second cousins helps. Each step out on the family tree cuts the expected dna sharing roughly in half. The table below gives a quick view of how those different cousin relationships line up.
| Cousin Type | Average Dna Shared | Typical Range (cM) |
|---|---|---|
| First cousin | 12.5% | Approximately 396–1397 cM |
| Half first cousin | 6.25% | Roughly 156–979 cM |
| First cousin once removed | 6.25% | About 220–680 cM |
| Second cousin | 3.125% | Approximately 46–515 cM |
| Third cousin | 0.78% | Around 16–110 cM |
When you compare a specific match against these patterns, you gain a better sense of how flexible each label can be. A match that shares dna near the high end for a first cousin could in some situations be a half sibling or a double cousin. A match near the low end might instead be a first cousin once removed or a second cousin who happens to share on the higher side of the range.
Why Your First Cousin Dna Sharing Might Not Match Expectations
Even when you know how much dna do you share with a first cousin? on average, real life results often surprise people. Several factors can push your shared percentage or centimorgan count away from the textbook value.
Random Recombination
The biggest driver is simple randomness. During each meiosis event, the paired chromosomes from a parent exchange segments at scattered crossover points. One sibling might inherit a long stretch of chromosome from a shared grandparent while another sibling skips that stretch entirely. When those siblings have children, their children inherit different mixes again, which shifts the final dna overlap between cousin pairs.
Pedigree Collapse And Double Relationships
Some families include marriages between relatives or repeated links between the same lineages. This pattern, sometimes called pedigree collapse, means that two people can be related in more than one way. Two first cousins might also be second cousins through another branch, or their grandparents might themselves be related. In those cases the expected dna sharing rises above the usual figure.
Test Company Differences
Testing companies use slightly different chips, segment thresholds, and algorithms to define what counts as a shared segment. One company might trim away shorter segments to reduce noise; another might include them. These decisions nudge reported cM totals up or down and explain small discrepancies between providers.
Using First Cousin Dna Sharing In Genealogy Research
Understanding how much dna you share with a first cousin helps you interpret match lists and build a more accurate tree. A match that fits the first cousin range supports the idea that your parents really do share the same set of grandparents or the same extended branch that family stories describe.
Many people pair centimorgan charts with careful record work. By using dna sharing as one clue among many, you avoid overreading small swings in percentages that fall well inside the normal range for first cousins and keep your focus on patterns that repeat across dna results and paper documents.
Key Takeaways About First Cousin Dna Sharing
For practical purposes you can treat first cousins as sharing about one eighth of their dna. The expected overlap is 12.5%, with test results usually landing somewhere between a little over 7% and almost 14%. That range reflects the random shuffling of chromosomes and the technical quirks of modern tests rather than any mismatch in the underlying relationship.
When you look at a match list and wonder about that first cousin label, check the reported centimorgan value and see whether it falls inside the familiar range. Then look at ages, locations, and paper records to confirm how that person fits into your tree. With that mix of dna data and records, the simple question how much dna do you share with a first cousin? turns into a practical tool for sorting real genetic connections.