November 30, 2022
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Can You Share DNA and Not Be Related?

Can You Share DNA and Not Be Related? Read on to find out the correlation between being relatives with someone and sharing DNA.
Tomohiro Takano

Can You Share DNA and Not Be Related? 

When you upload the results you receive from a DNA heritage test, the amount of information you get back can be overwhelming.

You may wonder if you’re really related to all of those matches, or if the accompanying information is incorrect. On the flip side, you might get almost no ancestry information back — no connections to trace on either family tree. That can’t be right, can it?

While FDA-approved DNA testing kits are held to a high medical standard and mistakes are rare, it is possible for human error or the contamination of a sample to affect results. However, it’s more likely that gaining a better understanding of how DNA testing works will provide the right answers. 

How Does a DNA Heritage Test Work?

Genetic testing was once reserved for learning critical medical information, but today DNA testing technology is available to everyone. A renewed interest in genealogy in people of all ages has sparked interest in over-the-counter consumer testing kits.

The ancestry features available through DNA testing make it possible to discover who your ancient genetic groups are and how they migrated across the globe. Most accurate DNA test kits also provide an option for entering your information into a database to help you discover living relatives that you may never have known about.

People use DNA ancestry tests to reunite with family members, locate biological parents, and enjoy learning about their family’s place in history. By isolating and analyzing a single strand of DNA in a sample of saliva, researchers can reveal more about your history — and your future — than you’ve ever imagined.

Researchers use the Y chromosome, which is passed to male children by their fathers, to track paternal ancestry. Maternal ancestry is tracked through mitochondrial DNA. Mothers pass mitochondrial DNA to all of their children regardless of sex. This information provides insight into possible relatives from both sides of your family.

Understanding the Role of Genetics

Every cell of every living organism contains deoxyribonucleic acid, or DNA. It’s the genetic material that determines everything from our eye color to the diseases we’re at risk of developing. DNA is self-replicating, meaning it passes its information on from one generation to the next.

There are millions of genetic building blocks, also known as base pairs, in the human body. The human genome is made up of about 3 billion base pairs, and 99.9% of that genetic material is identical to other humans. It’s been recently discovered that humans share almost 98.8% of their genes with chimpanzees and about 90% with cats.

Sharing genetic material with other animals or even plants doesn’t mean we’re related to them. DNA is a chemical building block that accounts for a large portion of the genetic material in every living thing. When you understand that, it’s no surprise that humans share at least some DNA with so many different types of organisms.

DNA Pieces vs. Segments

If we share so much genetic material with so many other things, how can even the most accurate DNA results be considered reliable?

For the purposes of medical or ancestry testing, researchers analyze the .01% of DNA that’s unique within all humans. It’s possible to share very small pieces of DNA within that .01% and still not be related.

Coincidentally matching DNA pieces are not the same as shared DNA segments. Matching segments are located on specific locations within an individual chromosome and are measured in centimorgans (cMs). Every human has 22 numbered chromosomes and approximately 6,800 centimorgans within those chromosomes.

For example, a match labelled “38 cMs across five segments” means two individuals share 38 of the same centimorgans and five segments of their DNA. Testing kits that include a chromosome browser for comparing DNA matches provide the most accurate DNA results.

If you didn’t use a test that includes a chromosome browser, you can upload your raw data to Genomelink for further comparison.

Every shared DNA segment is probably inherited from a common ancestor. It’s typical to share multiple DNA segments with our closest relatives but only one or two with a more distant cousin.

No specific number of DNA segment matches are required to qualify as a “match.” Closer relationships like parents or siblings will share more, while distant relatives will share less. That’s why it is possible to share dozens of small pieces of DNA with people who aren’t related to you.

Consequently, the most accurate DNA tests will focus on the size of shared segments to determine how far back in the family tree a common ancestor might be.

How Are Matching DNA Segments Sized?

To fully understand how genetic connections are determined, it’s helpful to understand the general guidelines that determine “large” or “small” matches. In general, the guidelines are:

  • Longer than 100 centimorgans: very large matching sample
  • Between 50 and 100 cMs: large match
  • Between 12 and 50 cMs: medium match
  • Less than 12 cMs: small match

If you share large or very large matches with another person, it means your common ancestor is only a few generations removed.

The smaller the matching segment, the farther back in the family tree the common ancestor is located. Sometimes, the ancestor is so far back it’s not possible to find anything about them through research.

How Much Is Enough to Be Related?

While there are no specific amounts that declare two people to be related, on average, we share about 12% of our DNA with first cousins, about 3% with second cousins, and less than 1% with third cousins.

It’s important to remember that sharing ancestry and sharing DNA is not always the same thing. It’s possible for two people to have a common ancestor in their distant history without having DNA matches.

As families branch off, migrate, and marry into new groups, DNA connections can be whittled down to practically nothing. Is your 104th cousin from your 6x great-grandfather your relative? Perhaps, technically speaking. But that relation may not bear out in DNA testing. 

It’s possible to share small amounts of DNA with people you’re not related to. It’s also possible to be related to people with whom you share no DNA. To help clarify your ancestry results and get the full benefit of your DNA testing, contact Genomelink for more information.

Tomohiro Takano
Tomohiro Takano
Co-Founder and CEO