Can a DNA test find a long-lost relative?

A woman in Kuching knows only that her father came from a small village in Sarawak, and that he had a brother. After her father passed away she never heard from that side of the family. She wonders if a cousin somewhere shares her last name, her eyes, her hands.

The story

Yes, a DNA test can find relatives. The way it works is simple. Every person inherits long stretches of DNA from their ancestors. Two relatives who share a recent ancestor also share long stretches of DNA from that ancestor. Scientists call this finding relatives from shared patterns of genetic variation (Mathieson and Scally, 2020). Ralph and Coop (2013) used exactly this idea, looking for long shared stretches, to study how closely people were related.

How close the match is tells how recent the shared ancestor is. Close family, such as full siblings, parents or first cousins, share a lot. Distant cousins share only a little, or none. Because DNA is shuffled in every generation, full siblings also differ in what they inherited from the same parents.

Knowing a relative is not the same as knowing your ancestry, but it is very useful. Mathieson and Scally note that knowing your relatives can help you pool information about shared ancestors, together with family papers, photos and stories.

There are two big conditions.

1. It needs a big pool of tests. A match can only be found if the relative has also taken a test and chosen to take part. In Malaysia, the pool is still growing. 2. It needs consent. A match is only shown to another person if both people have clearly said yes. Nobody is shown without their agreement, and anyone can say no, or change their mind, at any time. This is the rule planned for Akar Nusa's find-your-relatives service.

Where you can still see it today

In your own family: the cousins who look like you, with a shared grandparent.
In family reunions, where a long-lost branch is reconnected by a phone call or an old photo.
In your family-line story: it can tell you about a mother or father line, though it cannot name a relative.
On the find-your-relatives page planned for this site, where you will decide who can see you.

What DNA can tell, and what it cannot

DNA can show that two people are probably related, and roughly how closely, if they share long stretches of DNA.

It cannot say exactly how they are related, whether a story about a family is true, or why they lost touch. A mother line or father line match alone does not say two people are close relatives, because many people share a line. Very distant cousins may show no match at all.

DNA does not decide race, religion or bumiputera status. It also cannot give a legal family relationship: a match is a clue, and the family's own papers and talk finish the story.