Every autosomal DNA test result contains a hidden structure that most researchers never see. Each of your 22 chromosome pairs is a mosaic — a patchwork of segments inherited from different grandparents, stitched together at points where chromosomes crossed over during the reproductive process that produced you. That mosaic is fixed for life, it’s unique to you, and it contains the answer to a question that conventional genealogical research cannot answer: which segments of your DNA came from which grandparent?

Visual phasing is the technique that extracts that answer. It is demanding, technically sophisticated, and requires specific circumstances to apply. But when those circumstances exist and the method is applied correctly, it produces something no other genealogical tool can: a chromosome-level map showing exactly which grandparent contributed each segment of your DNA — and, by extension, which of your DNA matches connect through each ancestral line.

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What Visual Phasing Does

Visual phasing is a process by which the DNA of three siblings is assigned to each of their four grandparents using identified recombination points, without needing to test any grandparents directly. The process works because siblings inherit their DNA from the same parents, but they don’t inherit the same segments. At conception, each parent’s chromosomes undergo a process called recombination — crossover points where the two copies of each chromosome exchange segments — before being passed to a child. The crossover points differ between children, which is why siblings are genetically distinct even though they share the same parents.

Comparing three siblings’ DNA against each other allows a researcher to identify where each sibling’s chromosomes switched from one grandparent’s DNA to the other. By mapping these crossover points systematically across all 22 chromosomes, the researcher can identify which segment of which chromosome came from which of the four grandparents. The end result is a color-coded chromosome map in which every segment is labeled with its grandparental origin.

Visual phasing is a technique developed by Kathy Johnston and Randy Whited which allows assigning DNA segments of three full siblings to all of their four grandparents by comparing their crossing-over positions on a specific chromosome. It’s even possible to reconstruct nearly complete DNA profiles of your ancestors using this technique, if you have five, six or more siblings.

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Why It Matters

The practical genealogical value of a phased chromosome map is substantial. Once you know which segments of your DNA came from your maternal grandmother versus your maternal grandfather, every new DNA match can be assigned to the correct grandparent line the moment you check which segments they share with you and where those segments fall on your phased map.

For researchers working on a specific brick wall — a line where the documentary record runs out — this is transformative. Rather than sifting through thousands of matches trying to determine which side of the family each one connects through, a phased map makes the connection visible: a match who shares a segment in a chromosome region mapped to your paternal grandmother’s DNA almost certainly connects through her line. The search space shrinks from your entire family tree to one quarter of it.

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What You Need

Three siblings who have all taken autosomal DNA tests is the standard requirement. The technique works because comparing three sibling pairs (A vs B, A vs C, and B vs C) provides enough information to identify and resolve the crossover points on each chromosome. With a handful of genetic relatives, you can actually phase most of the 22 chromosomes using just two siblings, though this is considerably more difficult than three-sibling phasing. Two-sibling visual phasing relies more heavily on known cousin matches to resolve ambiguous crossover points.

Segment data from a platform that provides it — GEDmatch (for the chromosome comparison tools), FamilyTreeDNA, or MyHeritage. AncestryDNA does not provide segment data, so Ancestry-only testers must transfer their raw DNA to one of these platforms before beginning.

Chromosome comparison tools — specifically, the one-to-one chromosome comparison that shows graphically where two individuals share DNA and where they don’t. GEDmatch’s chromosome browser is the most commonly used for this purpose.

A mapping tool — either a spreadsheet or DNA Painter (discussed in depth in the following article in this series). Blaine Bettinger’s spreadsheet method is widely recommended as the easier approach for beginners, though semi-automated Excel spreadsheets developed by the visual phasing community on Facebook are also available.

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The Process in Overview

Visual phasing works chromosome by chromosome. For each of the 22 autosomal chromosomes, the process follows the same sequence:

First, generate the three pairwise chromosome comparisons between the siblings (A-B, A-C, B-C) and identify the crossover points — the locations where the comparison switches from shared to not-shared or vice versa. These crossover points correspond to locations where a parent’s chromosomes recombined during that sibling’s conception.

Second, align the crossover points across all three comparisons and determine which parent each crossover belongs to. Because you’re looking at the same chromosome across three siblings, the patterns interact in ways that reveal which parent contributed each segment.

Third, assign grandparent labels to each segment. Once you know which parent each segment came from and which crossover belongs to which parent, you can begin assigning grandparent labels using known relatives — a cousin on one grandparent’s line whose shared segment falls in a specific region confirms which grandparent owns that region.

Fourth, use known DNA matches to resolve any remaining ambiguous segments. Most chromosomes will have a few regions where the sibling comparisons alone can’t determine which grandparent is the source; known cousin matches fill these gaps.

Starting with the highest-numbered chromosomes, which are shortest and have fewer crossover points, is generally recommended for beginners. Chromosomes 22 and 21 are the natural starting points; chromosomes 1 and 2, being the longest, are typically the most complex to phase.

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Tools and Resources

The visual phasing community has developed several tools that make the process more tractable:

DNA Painter (dnapainter.com) has built-in support for chromosome mapping and allows you to paint segments from known relatives to build a phased map incrementally, even without full three-sibling visual phasing. The Cluster Mapper tool on DNA Painter can import autocluster results and map them directly onto a chromosome map.

The Visual Phasing Facebook group maintains shared resources including Steven Fox’s automated Excel spreadsheet, which semi-automates the crossover identification process.

GEDmatch’s chromosome browser remains the most commonly used tool for generating the comparison graphics that visual phasing depends on, since it allows comparison of any two uploaded kits with full graphics and position data.

Blaine Bettinger’s five-part series on The Genetic Genealogist blog (thegeneticgenealogist.com) is the most comprehensive step-by-step guide available and is the standard starting resource for researchers learning the technique.

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Realistic Expectations

Visual phasing is not a beginner technique. Even experienced genetic genealogists find it time-consuming and occasionally ambiguous — chromosomes with many crossover points, half-sibling relationships, or segments where the comparison data from two testing companies doesn’t align cleanly can all create difficulties.

Neither manual nor automated methods solve all problem areas on chromosomes where crossover points between two siblings don’t line up. The misalignment could be the result of fuzzy ends of segments, comparing DNA results from two different testing companies, or a double crossover.

It is also worth being clear about what visual phasing cannot produce. It can map your own 44 autosomal chromosomes (22 pairs) to their grandparental origins with high accuracy when conditions are favorable. It cannot extend that mapping to your matches’ chromosomes, nor can it identify which specific great-grandparent or more distant ancestor within a grandparent’s line contributed a specific segment. The result is a grandparent-level map, not an ancestor-level one.

For researchers who have the necessary sibling test data and the patience to work through the process methodically, however, visual phasing produces a result that is genuinely irreplaceable — a permanent, chromosome-level attribution of your DNA to the four people who were each responsible for one quarter of it.

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For Dillman Researchers

Visual phasing addresses one of the most persistent challenges in Dillman surname research: distinguishing between multiple unrelated family groups that share the surname or a close variant. The DFA’s Family Group Sheet database documents 199 distinct family groups, many of which have no established connection to each other beyond a shared name and, often, a shared region of German origin. A DNA match who carries the Dillman surname is not necessarily connected through a Dillman ancestor at all — the connection could just as easily run through an entirely different, non-Dillman line shared by coincidence.

For a researcher who has phased their chromosomes and identified which segments trace to a known Dillman grandparent, this ambiguity disappears. Any new match sharing a segment within that grandparent’s mapped region can be evaluated specifically as a candidate Dillman connection, while matches falling outside it can be set aside for that particular research question, regardless of what surname they carry. This is a meaningfully different, and more reliable, filter than surname matching alone.

The technique is also well suited to testing suspected connections between family groups. Several entries in the DFA’s ongoing DNA outreach work identify pairs of family groups suspected of sharing a common ancestor — Groups 20 and 52, or Groups 53 and 54, for example — where the hypothesis currently rests on geographic and circumstantial evidence alone. A DFA member with three tested siblings and a phased chromosome map is positioned to evaluate a specific match from one of these groups with real precision: does the shared segment fall within a region already attributed to a known ancestral line, or does it point to an unexplained connection worth investigating further?

Combined with the clustering techniques covered earlier in this series, visual phasing gives a Dillman researcher two of the most powerful tools available for making sense of a large, tangled match list: clustering to sort matches into ancestral-line groups, and phasing to confirm, with chromosome-level precision, which grandparent each group actually traces to.

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Sources

Bettinger, Blaine. “Visual Phasing: An Example (Part 1 of 5).” The Genetic Genealogist. November 21, 2016. thegeneticgenealogist.com.

Genetic Genealogy Girl. “Visual Phasing.” Accessed July 2026. geneticgenealogygirl.com.

Family History Fanatics. “Can You Do Visual Phasing With Only Two Siblings?” April 21, 2021. familyhistoryfanatics.com.

Family Locket. “Chromosome Mapping – Visualize Your DNA and Identify the Ancestors Who Passed It On To You.” familylocket.com.

Albuquerque Genealogical Society. “Visual Phasing: An Update.” October 6, 2021. abqgen.org.

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Note: This post was researched and written with the assistance of artificial intelligence (AI). All content has been reviewed for accuracy as of September 2026. All errors are the responsibility of the author.