Why siblings share about 50%
You get half your DNA from each parent. But which half is random: each parent passes on a different shuffled mix to each child. So two siblings each get half of mom's DNA and half of dad's, and those halves overlap by about half. Add it up and full siblings share about 50% on average.
The key word is average. Because the shuffle is random, real siblings vary. A large study of more than 4,000 sibling pairs found sharing ranged from about 37% to 62%. You can be noticeably more alike genetically to one sibling than to another.
Parents are different. You always share almost exactly 50% with each parent, because you got exactly one copy of each chromosome from them.
Siblings by type
| Siblings | Shared DNA (average) | On a DNA test (average cM) | Typical cM range |
|---|---|---|---|
| Identical twins | ~100% | ~3,500+ (shown as a full match) | n/a |
| Full siblings, fraternal twins | 50% | 2,613 | 1,613–3,488 |
| Three-quarter siblings* | 37.5% | ~2,200 | overlaps full and half |
| Half siblings | 25% | 1,759 | 1,160–2,436 |
| Step-siblings | 0% | no match | n/a |
*Three-quarter siblings share one parent, and their other parents are siblings, for example two brothers who each had a child with the same woman. Source for cM figures: Shared cM Project v4.
Why the cM number isn't "50%" of the total
DNA tests count shared stretches of DNA in centimorgans (cM). A parent and child share about 3,485 cM, which is the "half" mark. Full siblings show about 2,613 cM, which looks like less than half. That's because full siblings also have fully identical regions, stretches where they match on both copies of a chromosome, and most tests count those only once. Count them twice and you're back at about 50%.
Full or half siblings? How to tell
The totals overlap: about 1,700 to 2,400 cM could be either. The tie-breaker is those fully identical regions. Full siblings have lots of them (about a quarter of their DNA). Half siblings have almost none. 23andMe shows this directly, and tools that compare raw DNA files can calculate it. Our sibling DNA test guide walks through it.
Half sibling, grandparent or aunt?
All three share about 25% of your DNA, or around 1,750 cM, so a DNA total alone can't tell them apart. Ages usually can. That's also why donor-conceived people often discover half siblings through consumer DNA tests: the match shows up with a number that only fits a handful of relationships.
When this really matters
Some sperm donors have dozens of offspring, and adoptions or family secrets mean some half siblings don't know about each other. If you're donor-conceived or adopted and starting something serious with someone, a quick DNA comparison isn't paranoid. It's practical.