If you’ve taken a DNA test or you’re sorting out family relationships, you’ve probably wondered about the numbers. Full siblings share about 50% of their DNA — but what about half siblings? The answer: half siblings share roughly 25% of their DNA on average, though the real number varies more than most people expect.
Let’s break down what that 25% actually means, why it varies, and how it compares to other relationships.
The Short Answer: About 25%
Half siblings — two people who share one biological parent but not the other — inherit half as much shared DNA as full siblings. Since full siblings average 50% shared DNA, half siblings average 25%.
In the centimorgan units that DNA testing companies use, half siblings typically share between 1,160 and 2,436 centimorgans (cM), with an average around 1,750 cM. For comparison, full siblings average about 2,600 cM. These ranges come from large datasets of tested relatives compiled by genetic genealogists, including the widely cited Shared cM Project data on DNA Painter.
Why the Number Varies So Much
Here’s the part that surprises people: DNA inheritance is random. You get exactly 50% of your DNA from each parent, but which 50% is a roll of the genetic dice. Two half siblings might share 18% or 32% — both are completely normal.
Think of it like shuffling two decks of cards. Each child gets a random half of the shared parent’s deck. Sometimes those random halves overlap a lot; sometimes they barely overlap. The average works out to 25%, but individual pairs land somewhere in a bell curve around that average.
This randomness is also why full siblings — who average 50% — can actually range from about 38% to 61%. The same principle applies at every level of relatedness.
How Half Siblings Compare to Other Relationships
The 25% figure puts half siblings in interesting company. Here’s how they stack up:
- Parent/child: 50% (exactly — you get half your DNA from each parent)
- Full siblings: ~50% average (range ~38–61%)
- Half siblings: ~25% average (range ~17–34%)
- Grandparent/grandchild: ~25% average
- Aunt/uncle and niece/nephew: ~25% average
- First cousins: ~12.5% average
- Half aunt/uncle: ~12.5% average
Notice something interesting: half siblings, grandparents, and aunts/uncles all average 25%. DNA tests can sometimes struggle to distinguish between these relationships without additional context like ages. A grandparent and a half sibling look genetically similar — the testing company needs your family tree details to tell them apart.
Maternal vs. Paternal Half Siblings
People often ask whether it matters which parent is shared. Genetically, the autosomal DNA percentage is the same either way — about 25%. But there are two small differences:
- Mitochondrial DNA: Passed only from mother to child. Maternal half siblings share identical mitochondrial DNA; paternal half siblings don’t share any.
- X chromosome: Fathers pass their single X chromosome to all daughters but to no sons. So paternal half sisters always share their father’s entire X chromosome, nudging their total slightly higher on average than paternal half brothers.
These are refinements, though — the headline number stays around 25% regardless.
What About “Three-Quarter Siblings”?
Here’s a fun edge case. If two siblings have children with the same partner (or with two siblings), their children are “three-quarter siblings” — genetically between half and full siblings, averaging about 37.5% shared DNA. It’s rare, but genetic genealogists encounter it.
Can DNA Tests Tell Half Siblings from Full Siblings?
Yes, reliably. The ranges barely overlap: half siblings top out around 34% while full siblings bottom out around 38%. Testing companies like AncestryDNA and 23andMe use these ranges to predict relationships, and they’re very good at the sibling vs. half-sibling distinction.
What tests can’t always do is distinguish between relationships at the same level — half sibling vs. grandparent vs. aunt/uncle, as mentioned above. That’s where your known family information fills in the gaps.
Common Situations Where This Comes Up
Adoption and donor conception
People who discover donor siblings or biological family through DNA testing often find half siblings they never knew about. The ~25% match is usually the giveaway.
Blended families
Kids in blended families sometimes wonder about the “real” difference between half and full siblings. Genetically it’s 25% vs. 50% — but family bonds don’t follow percentages, and plenty of half siblings are closer than many full siblings.
Genealogy research
If you’re building a family tree, understanding these percentages helps you interpret unexpected DNA matches. A match around 1,750 cM with someone your age is very likely a half sibling.
Does Sharing 25% of DNA Mean Anything for Health?
Not in any practical sense for the siblings themselves. Shared DNA matters most for inherited health conditions when it comes to parent-child transmission. Between siblings, the shared percentage is more of a genealogical curiosity than a medical factor — though knowing your family health history from both parents’ sides is always worthwhile. The National Human Genome Research Institute’s DNA fact sheet is a solid primer if you want to go deeper.
The Bottom Line
Half siblings share about 25% of their DNA on average — half the ~50% that full siblings share. The actual number varies from roughly 17% to 34% because inheritance is random. It’s the same average as a grandparent or aunt/uncle, which is why DNA tests sometimes need your family context to sort out exactly how you’re related.
Curious about more surprising numbers from science? Check out how many extinction events Earth has survived — another case where the figures are bigger than you’d guess.
How DNA Inheritance Actually Works
To really grasp the 25% figure, it helps to understand how DNA inheritance works at the mechanical level. You have 23 pairs of chromosomes — one set from each parent. When your body makes sperm or egg cells, it randomly picks one chromosome from each pair (a process called recombination shuffles them further). That’s why siblings differ: each child gets a different random sample of each parent’s DNA. Half siblings share only one parent’s contribution, so only that parent’s random sampling creates overlap — hence roughly half the sharing of full siblings. The randomness of recombination is exactly why the range is so wide.
How Testing Companies Calculate Your Matches
When AncestryDNA or 23andMe says someone is your half sibling, they’re not sequencing your whole genome and counting percentages. They compare hundreds of thousands of genetic markers and measure the total length of matching DNA segments in centimorgans. Long unbroken matching segments indicate recent shared ancestry; the total determines the relationship prediction. The companies then compare your total against reference ranges from tens of thousands of known relationships. It’s statistics, not a paternity test — which is why they report predicted relationships with confidence levels rather than certainties.
Half Siblings vs. Other 25% Relationships: Telling Them Apart
Since half siblings, grandparents, and aunts/uncles all average 25%, how do testers distinguish them? Three clues:
- Age: A 25% match your own age is almost certainly a half sibling; one 50 years older is likely a grandparent or aunt/uncle.
- Segment patterns: Grandparent-grandchild matches show a distinctive pattern — the grandchild’s matching segments come entirely from one parent’s side. Half siblings’ segments are distributed across both sides of the shared parent.
- Shared matches: Looking at who else you both match with triangulates the relationship. A half sibling will share matches on the shared parent’s side only.
Genetic genealogists use these patterns routinely to solve unknown-parentage cases — it’s how many adoptees identify birth families.
Do Half Siblings Share Traits?
People often wonder whether the 25% sharing shows up in looks or personality. Sometimes strikingly — half siblings frequently resemble each other, since the shared parent’s genes influence both. But it’s a lottery: two half siblings might look like twins or like strangers. As for personality, behavioral genetics suggests shared genes influence temperament modestly, but upbringing and individual experience dominate. The DNA percentage predicts genetic overlap, not how similar two people feel — and every family has stories that prove it.
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