Sibling DNA test

Find out whether two people share both parents, one parent, or neither.

Answers
Full siblings, half siblings, or not related
Who takes part
Two or more possible siblings, and a parent if available
Samples
Cheek swabs
Result
A siblingship index: how much more likely one answer is than the other
Court use
With a witnessed collection

How it works

On average, full siblings share about half their DNA and half siblings about a quarter. The lab compares the markers of each person and reports how likely they are to be full siblings, half siblings or unrelated.

Make the result stronger

  • Add a known parent. If the siblings share a mother and the question is the father, testing the mother lets the lab set aside what came from her.
  • Add more siblings where you can.

Brothers and sisters who may share a father

  • Two brothers: a Y-chromosome test shows whether they share a male line, though not which man in it.
  • Two sisters: an X-chromosome test can show whether they share a father, because a father passes the same X chromosome to every daughter.
  • A brother and a sister: the standard sibling test.

Reading the result

A sibling result is a likelihood, not a yes or no. A siblingship index above 1 supports the relationship, and the higher it is, the stronger the support. Telling full from half siblings is less clear-cut than a paternity test, which is why a known parent helps.

Questions

Is a sibling test as certain as a paternity test?

No. Siblings share a varying amount of DNA, so the answer is a likelihood. Testing a known parent, or more siblings, makes it clearer.

Can a sibling test show who the father is?

It can show whether two children share a father, which helps when the father can't take part. See tests without the father.

Do the siblings need to be in the same place?

No. Each person gives a sample where they live.

Guides for this test

  • Can you do a paternity test without the father?Yes: a lab can compare the child's DNA with the possible father's close relatives, and testing both of his parents gives the clearest answer. If he has died, a stored sample held by a medical facility, coroner or medical examiner may also work.
  • How do you read paternity test results?Look first at the probability of paternity and the conclusion. A result of 0% means the man tested is excluded as the biological father; 99% or more, backed by a large combined paternity index, means he is very likely the father.
  • Can a paternity test be wrong?Rarely, and most often because of a sample problem, such as a mix-up or contamination, or because the man tested is closely related to the real father. Identical twins are the one case a standard test can't settle.

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