How much DNA do you typically share with a cousin
On average, you share about 12.5% (roughly 1 in 8) of your DNA with a first cousin, since you both inherit about half of your DNA from each of your shared grandparents. This is a useful rule of thumb, but the actual amount can vary due to chance, family history, and whether the cousin is a close relative on one side or involves multiple branches of the family. The numbers are estimates and can differ between testing companies and testing methods, so treat them as informative ranges rather than fixed values.
How relationship closeness affects shared DNA
DNA inheritance is probabilistic: each parent passes about half of their autosomal DNA to each child, shuffled through recombination. Because of this shuffling, relatives typically share a predictable range of DNA rather than a single exact percentage. Closer relatives share more DNA on average, but every inheritance event is independent, so actual sharing varies. Understanding these averages helps you interpret DNA match estimates and set realistic expectations about what a match means for your family tree.
Key concepts you should know
- Autosomal DNA: the mixture of DNA from all ancestors that is used by most consumer tests for cousin and relative matching.
- Shared centimorgans (cM): a measure of genetic material length used to estimate closeness and predict relationship ranges.
- Inheritance recombination: the reshuffling of DNA from grandparents to parents to children, which explains variation around averages.
- Testing vendor differences: different companies use different chips and algorithms, which can change reported amounts of shared DNA.
- Identical by descent (IBD): DNA inherited from a common ancestor that matches on both copies of a chromosome segment.
- Triangulation: when multiple relatives match you and each other on the same segment, increasing confidence that the segment comes from a shared ancestor.
Typical DNA sharing ranges by cousin type
Relationship categories have expected ranges, based on large studies of inheritance patterns. Your exact shared DNA may fall within or outside these ranges due to recombination. Use these as reference points rather than strict cutoffs, and consider multiple relatives and shared segments for stronger conclusions.
| Relationship (on the same side) | Typical percent DNA shared | Typical centimorgan (cM) range | Source-backed note |
|---|---|---|---|
| Parent | About 50% | About 3,300 cM | Consistent across major vendors |
| Grandparent / Aunt or Uncle / Half-sibling | About 25% | About 1,200–1,400 cM | Ranges vary slightly by vendor |
| First cousin | About 12.5% (1/8) | About 575–670 cM | 23andMe and other tests align closely |
| First cousin once removed | About 6.25% (1/16) | About 100–250 cM | Ranges depend on generational gap |
| Second cousin | About 3.125% (1/32) | About 50–200 cM | Shared DNA often small but detectable |
| Second cousin once removed | About 1.5% (≈1/64) | About 50–120 cM depending on age | Useful for more distant genealogy |
Why your DNA with a cousin may differ from the average
Even full first cousins typically share between about 470 and 670 centimorgs, with an average near 575 cM, reflecting normal variation in which segments are inherited. Identical twins share nearly 100% of their DNA, while full siblings average about 50% but can technically share more or less due to recombination. Half-siblings, grandparent–grandchild, and aunt–nephew pairs usually average about 25%, but observed values can differ. Parent–child and parent–parent relationships are the most consistent, often near 50%, whereas more distant cousins show greater overlap with ranges from other relatives and more sensitive thresholds at testing companies.
What affects the exact amount you share
- Which chromosomal segments are inherited: recombination shuffles grandparents’ DNA, so cousins may inherit different portions of the same ancestral segments.
- Family history of endogamy or close marriages: this can increase shared DNA because ancestors are more closely related.
- Testing company’s chip and calling algorithm: different thresholds and phasing can change reported shared cM and percentage.
- Whether the relationship involves multiple family branches: double relationships (e.g., double first cousins) can raise average shared DNA toward that of closer relatives.
- Phasing quality and whether DNA is clipped at low levels: very small segments may be ignored or counted differently depending on the test vendor.
How to use this information for genealogy and matching
Use average numbers as a baseline while relying on actual shared cM values reported by your testing service. Many tools and shared cM calculators convert a reported cM range into likely relationship predictions, helping you confirm or hypothesize connections. When you have multiple matches, triangulate on segments and look for shared surnames or paper trails to build confidence. Keep in mind that adoption, misattributed parentage, or complex family history can shift observed DNA, so combine genetic data with traditional records for the most reliable conclusions.
Frequently asked questions about cousin DNA sharing
- Why do some first cousins share less DNA than others? Recombination varies each generation, so the exact segments passed to each cousin differ, producing variation around the 12.5% average.
- Can a first cousin share more than 25% of DNA with me? Not typically; values much above 25% may signal a closer relationship (such as a grandparent or half-sibling) or a data/phasing artifact, and warrant further investigation.
- Do ethnicity estimates affect DNA matching with cousins? No, ethnicity estimates are population-level and do not determine relative-matching; shared DNA matching depends on specific inherited segments, not ancestry composition.
- How many shared centimorgans confirm a first cousin relationship? Most first cousins share roughly 575–670 cM, but typical ranges often cited are about 500–700 cM depending on the testing vendor.
- Should I consider tree confidence when interpreting shared DNA with cousins? Yes, combine shared DNA with documentary evidence, consistent surnames, and migration patterns to build a reliable conclusion about the relationship.
Common pitfalls and how to avoid them
Avoid treating a single percentage or cM value as proof of exact relationship without considering ranges and testing differences. Don’t ignore small matches that might be more distant cousins or endogamous relatives. Be cautious when inferring relationships solely from one testing vendor without cross-checking averages, and remember that phasing quality and data clipping vary across companies. Triangulation and multiple cousin matches generally give a clearer picture than any single match alone.