Are we related? Compare two raw DNA files
Two people’s raw DNA files can show how closely they are related: the more DNA markers they match on, the closer the relationship. Drop both files below and the tool works out a kinship coefficient with the published KING method and names the closest fitting relationship, from the same person to unrelated.
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How it works
Over the SNPs both files read on chromosomes 1-22: kinship = 1/2 − (hetA + hetB − 2·hetBoth + 4·opposite) / (4·min(hetA, hetB)) hetA, hetB = SNPs where each person has two different letters hetBoth = SNPs where both do, opposite = SNPs like AA vs GG (IBS0) > 0.354 same person or identical twin · 0.177-0.354 first degree · 0.088-0.177 second · 0.044-0.088 third · below unrelated First degree: almost no opposite SNPs (under 0.1 × what unrelated people show) = parent and child, else full siblings
A kinship of 0.5 means the same DNA, 0.25 a parent, child or full sibling, 0.125 a half sibling, grandparent, aunt or uncle, and 0.0625 a first cousin. Results get less sure the fewer SNPs the two files share, and third-degree and more distant cousins can look unrelated. This is not a legal paternity test, and only compare a file with that person’s permission.
Sources
- Manichaikul A, Mychaleckyj JC, Rich SS, Daly K, Sale M, Chen WM. Robust relationship inference in genome-wide association studies. Bioinformatics 2010;26(22):2867-2873. doi:10.1093/bioinformatics/btq559
Worked example
The numbers from the source, run through this tool:
Kinship coefficient: 0.250(Full siblings)
KING-robust eq. 11 by hand: 0.5 − (3000 + 3000 − 2×1700 + 4×100) / (4×3000) = 0.5 − 3000/12000 = 0.250 (1st degree, 0.177-0.354). IBS0 = 100/10000 = 1%; unrelated IBS0 ≈ 0.3×0.3/2 = 4.5%, ratio 0.22 ≥ 0.1 → full siblings
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- Can I compare two raw DNA files to see if we are related?
- Yes. Download the raw data from each person’s DNA company and drop both files above. The tool lines up the SNPs both files read and works out how much DNA the two people share.
- What is a kinship coefficient?
- It is the chance that a DNA letter picked from one person and the same spot picked from the other came from a shared ancestor. It is 0.5 for the same person, 0.25 for a parent or full sibling, 0.125 for a half sibling and about 0 for unrelated people. Twice the kinship is roughly the share of DNA in common.
- Does a 25% DNA match mean we are half siblings?
- About 25% shared DNA (kinship near 0.125) is second degree. Half siblings, a grandparent and grandchild, and an aunt or uncle with a niece or nephew all share about that much, so comparing DNA alone cannot tell them apart. Ages and family history help.
- How does it tell a parent from a full sibling?
- Both share about half their DNA. A parent and child share at least one letter at every SNP, so opposite pairs like AA vs GG almost never happen. Full siblings have spots where they inherited nothing in common, so some opposite pairs show up.
- Can I compare files from different companies?
- Yes, but different companies read different SNPs, so fewer SNPs line up and the result is less sure. Files from the same company, or chip versions, compare best.
- Is this a paternity test?
- No. It is a free comparison of two raw files and not a legal or court paternity test. Labs that do legal tests check who gave each sample.
For information only. This is not medical advice and not a diagnosis. Talk to a doctor about any result that worries you.