How much DNA do brothers and sisters share?

Casey Randall AlphaBiolabs

By Casey Randall, Head of Genetics at AlphaBiolabs
Last reviewed: 11/10/2023

In this article, we discuss how brothers and sisters share DNA, and what can be learnt from a sibling DNA test.

How much DNA do brothers and sisters share?

How much DNA brothers and sisters share depends on their relationship to each other, and whether they have one or both biological parents in common.

Full siblings who have the same biological mother and father are known as first-degree relatives and share approximately 50% of their DNA.

Half siblings who have one biological parent in common – their mother or father – are known as second-degree relatives and share approximately 25% of their DNA.

By way of comparison, identical twins are always the same gender (both male or both female) and share near-identical DNA profiles, barring some minor variations which are very difficult to detect.

What can be learnt from a sibling DNA test?

A sibling DNA test can be used to determine whether two or more individuals are full siblings or half siblings and can be used on males and females.

For a sibling DNA test, cheek swabs are used to collect DNA samples from each sibling taking part in the test.

For the most conclusive result, the laboratory will also request that the biological mother’s DNA sample be submitted for testing, if possible.

Expert geneticists will then analyse and compare the DNA samples to determine the genetic profile of the possible siblings and calculate a sibling DNA index or likelihood ratio.

  • A likelihood of greater than 10 supports that the tested individuals are likely to be related
  • A likelihood of less than 0.1 is not supportive, meaning the individuals are unlikely to be related
  • A likelihood which falls between 0.1 and 10 is considered inconclusive, which means a relationship cannot be determined based on the statistical probability obtained from the analysis

Where can I buy a sibling DNA test?

It’s easy to order your peace of mind Sibling DNA Test online now for just €229, with results in 2-3 working days.

Simply place your order online and we will ship your test kit out to you in discreet, plain packaging, for full confidentiality.

Your home test kit will contain everything you need to collect your samples and return them to our ISO 17025-accredited laboratory for analysis by our in-house geneticists.

For confidential advice on whether this test is right for you, call our friendly and discreet Customer Services team on 01 402 9466 or email

Sibling DNA Test

Order our home Sibling DNA Test online now for €229.

Casey Randall AlphaBiolabs

Casey Randall

Head of Genetics at AlphaBiolabs

Casey joined the AlphaBiolabs team in 2012 and heads up the DNA laboratory.

An expert in DNA analysis and a member of the International Society for Forensic Genetics (ISFG), Casey holds an MSc with Distinction in DNA Profiling and a First-Class BSc with Honours in Forensic Science.

Casey is responsible for maintaining the highest quality testing standards, as well as looking for ways to further enhance the service that AlphaBiolabs provides and exploring new and innovative techniques in DNA analysis.

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