Uncles and nephews share about 25% of their DNA on average. In practice, the actual number ranges from roughly 17% to 34% because of how DNA shuffles between generations.
We explain why the range varies, what the numbers mean in centimorgans, and what to do when your results look unexpected.
How DNA Moves From Parent to Uncle to Nephew
Your uncle shares about 50% of his DNA with your parent because they are siblings. You inherited about 50% of your DNA from that parent. So on average, you and your uncle overlap on about 25% of your genome.
But that 25% is an average, not a guarantee. During reproduction, each parent’s DNA goes through recombination, a shuffling process that mixes segments before passing them on. This means the specific segments you inherited from your parent may overlap more or less with the segments your uncle got from the same parents.
That is why real test results fall anywhere from about 1,200 cM to 2,400 cM for a full uncle-nephew relationship.
Percentages vs. Centimorgans
DNA testing services report shared DNA in both percentages and centimorgans (cM). Percentages give you a quick summary. Centimorgans tell a more precise story because they account for where and how the shared segments fall across your chromosomes.
Here is why the distinction matters.
| Shared DNA | Centimorgan Total | Likely Relationship |
|---|---|---|
| ~25% | ~1,750 cM | Full uncle/nephew |
| ~12.5% | ~900 cM | Half-uncle/nephew |
| ~25% | ~1,750 cM | Grandparent/grandchild |
| ~25% | ~1,750 cM | Half-sibling |
Notice that several different relationships can produce similar numbers. A full uncle and a grandparent often share about the same amount of DNA with you. This is why we recommend looking at segment patterns and additional matches, not just one number.
Half-Uncles and Other Variations
If your uncle is a half-sibling to your parent, meaning they share one biological parent instead of two, the expected DNA overlap drops to about 12.5%. That translates to roughly 500 to 1,300 cM.
Half-uncle relationships can sometimes be mistaken for first-cousin relationships in test results because the centimorgan ranges overlap. If your results suggest a closer or more distant relationship than you expected, a half-sibling situation somewhere in the family tree is a likely explanation.
Step-uncles who are not biologically related to your parent will share no more DNA with you than any unrelated person would.
When Results Fall Outside the Expected Range
Sometimes the shared percentage lands well above or below the 17% to 34% range for a full uncle-nephew pair. A few things can explain this.
Endogamy occurs when a community intermarries over many generations. If your family comes from a small or isolated population, you and your uncle may share more DNA than the typical range suggests because your parents were already somewhat related before they had children.
Segment pile-up can inflate centimorgan totals without changing the actual relationship. Some chromosomal regions are passed down more consistently than others, which can make two people appear more closely related than they are.
Misattributed parentage is another possibility. If the uncle is not biologically related to your parent in the way you assumed, the numbers will reflect the true biological relationship, not the family story.
If your results seem off, we recommend testing additional relatives. Having your parent and your uncle both tested makes it much easier to confirm the relationship.
The Bottom Line
Uncles and nephews share about 25% of their DNA on average, but the real number varies because of how genetic recombination works. If your results fall between roughly 1,200 cM and 2,400 cM, the relationship is consistent with a full uncle-nephew match.
If the number seems too high or too low, do not jump to conclusions. Test another relative, check whether a half-sibling relationship is possible, and use centimorgan calculators to narrow the range of likely relationships.
