DNA Academy • Track 3: Segment Mapping

Understanding DNA Phasing: How to Separate Maternal and Paternal Chromosomes

Tired of false cousin matches and overlapping segments? Learn how DNA phasing sorts your raw data into maternal and paternal sides to solve deep genealogy brick walls.

When you take a commercial DNA test from AncestryDNA, 23andMe, or MyHeritage, your raw data provides a list of genetic letters (alleles) across your 22 pairs of autosomal chromosomes. However, standard testing produces unphased DNA—a combined mixture where your mother’s and father’s genetic contributions are lumped together into a single file.

DNA Phasing is the process of sorting those genetic letters into two distinct, parent-specific strands: one maternal chromosome and one paternal chromosome. Phasing is the single most powerful technique in genetic genealogy for eliminating false matches, mapping ancestral segments, and verifying your family tree with mathematical certainty.

💡 The Sock Drawer Analogy: Unphased vs. Phased DNA

Imagine receiving a box containing 46 mismatched socks belonging to two different people (your parents).

Unphased DNA is like looking into that unsorted box: you know all the socks are there, but you can’t immediately tell which sock belongs to whom.

Phased DNA is taking the time to separate every single pair and placing your mother’s socks in the left drawer and your father’s socks in the right drawer. Suddenly, matching a sock to a cousin becomes instant and error-free!

Why Unphased DNA Leads to False Cousin Matches

To understand why phasing matters, you first need to understand how commercial algorithms read your Raw DNA Data. At any given SNP (position on a chromosome), you inherit one nucleotide letter from your mother and one from your father (e.g., an A from Mom and a T from Dad).

In an unphased file, the testing company simply sees that you have A and T at that location—it doesn’t know which parent gave you which letter.

⚠️ The Danger of Small Matches: IBS vs. IBD

Without phasing, comparison algorithms sometimes create “frankenstein segments” by hopping back and forth between your maternal and paternal letters. This results in Identity by State (IBS)—a match that appears identical purely by statistical chance, not because you share a recent common ancestor.

Phasing guarantees Identity by Descent (IBD): an unbroken segment inherited directly from a genuine ancestor. Learn more about segment validation in our Understanding cM & Relationships Guide.

The 3 Primary Methods of DNA Phasing

Depending on which relatives you have tested and which platforms you use, there are three distinct ways to phase your genetic data:

Comparison of Phasing Methods

🔬 Pro-Tip: Testing a Parent is Worth 10x More Than Testing Yourself

If your biological parents are living, testing them is the single highest-ROI investment you can make in your genealogy research.

A parent’s test immediately phases your entire match list, eliminates thousands of false distant matches under 10 cM, and gives you access to an entire extra generation of genetic material that you didn’t inherit!

How to Phase Your DNA Data: Step-by-Step

Ready to phase your DNA and take control of your match list? Follow this simple workflow using free tools and testing platforms:

• 1

  Download Your Raw DNA Data

  Export your unphased raw data zip file from AncestryDNA, 23andMe, or MyHeritage. Follow our step-by-step Raw DNA Data Download Guide.

• 2

  Upload to GEDmatch

  Create a free account on GEDmatch and upload your raw file along with at least one parent’s raw file. Read our How DNA Uploads Work Guide for transfer tips.

• 3

  Run the GEDmatch Phasing Tool

  Use the free “Phasing” tool on GEDmatch. The system compares your kit with your parent’s kit and generates two brand new phased kits: one representing your pure maternal side and one for your paternal side.

• 4

  Analyze Phased Kits in Chromosome Browsers

  Run One-to-Many or One-to-One comparisons using your new phased kit numbers. Every match that appears on your phased maternal kit is guaranteed to belong to your mother’s lineage! Learn how to map these in our Chromosome Browser Guide and verify them with Triangulation Strategies.

What You Can Do Once Your DNA is Phased

Once you have phased maternal and paternal data, advanced genetic genealogy tools become 10x more effective:

• Paint Ancestral Chromosomes: Use tools like DNA Painter to assign specific chromosome segments to known great-grandparents with zero overlap confusion.
• Triangulate Distant Matches: Confidently verify shared segments under 10 cM down to specific ancestral couples. Use our DNA Relationship Probability Calculator to test candidate relationships.
• Reconstruct Deceased Parents: If you phase your data against your mother, GEDmatch can automatically construct a “Lazarus Kit” representing your deceased father’s DNA!