DNA Relationship Tools and Relationship Analysis
These projects are designed to help individuals interpret the amount of shared DNA (measured in centiMorgans, or cM) between two people who have taken a DNA test, such as those offered by AncestryDNA, 23andMe, MyHeritage, or FamilyTreeDNA.
What is the Shared cM Project?
The Shared cM Project is a collaborative effort led by genetic genealogist Blaine Bettinger. It collects and analyzes data from thousands of DNA test-takers to create a comprehensive chart that shows the average, minimum, and maximum amounts of shared DNA for various relationships. This helps people understand the likely relationship between themselves and their DNA matches.
How Does It Work?
When two people take a DNA test, the testing company compares their DNA and reports the amount of shared DNA in centiMorgans (cM). The Shared cM Project provides a reference chart that shows the typical range of shared cM for different relationships, such as:
- Parent/child
- Siblings
- Aunt/uncle
- Cousins (1st, 2nd, 3rd, etc.)
- More distant relatives
For example:
- Parent/child relationships typically share about 3,400 cM.
- Full siblings share approximately 2,600 cM on average.
- First cousins usually share around 850 cM.
Why Is It Useful?
The Shared cM Project helps people:
- Identify Relationships: By comparing the amount of shared cM with the chart, you can estimate how you might be related to a DNA match.
- Validate Family Trees: It can confirm or challenge relationships suggested by traditional genealogy research.
- Solve Mysteries: For adoptees or those with unknown parentage, it can provide clues about biological family connections.
Limitations
- Variability: The amount of shared cM can vary significantly, even for the same type of relationship (e.g., two first cousins might share 700 cM or 1,000 cM).
- Polygamy or Endogamy: In cases of repeated relationships (e.g., cousins marrying cousins), shared cM amounts may not fit the typical ranges.
- Half-Relationships: Half-siblings, half-aunts/uncles, etc., will share less DNA than full relationships.
How to Use the Shared cM Project
- Take a DNA test and get your results.
- Note the amount of shared cM with a match.
- Compare the shared cM to the project’s chart to estimate the relationship.
- Use additional tools (e.g., family trees, chromosome browsers) to refine your conclusions.
Where to Find the Chart
The Shared cM Project chart is freely available online. You can find it on Blaine Bettinger’s website, The Genetic Genealogist, or on platforms like DNA Painter, which provides an interactive version of the chart.
Different Systems for Matching cM Results to Relationships
There are several systems and tools available that help match cM results from a DNA test to a potential relationship or test match. These tools use the amount of shared DNA (measured in centiMorgans, or cM) to estimate how two individuals might be related:
1. Shared cM Project Chart
What It Is: A reference chart created by Blaine Bettinger that provides average, minimum, and maximum shared cM values for various relationships.
How It Works: You input the amount of shared cM between two individuals and compare it to the chart to estimate the possible relationship.
Where to Find It: Available on Blaine Bettinger’s website (The Genetic Genealogist) or integrated into tools like DNA Painter.
Strengths: Simple, widely used, and based on real-world data from thousands of test-takers.
Limitations: Provides ranges, so it may not pinpoint exact relationships without additional context.
2. DNA Painter
What It Is: An interactive tool that uses the Shared cM Project data to help visualize and interpret DNA matches.
How It Works: You input the shared cM value, and DNA Painter shows possible relationships and their probabilities. It also allows you to “paint” segments of shared DNA to visualize how they overlap with other matches.
Where to Find It: dnapainter.com
Strengths: Visual and interactive, making it easier to understand complex relationships.
Limitations: Requires manual input and some familiarity with genetic genealogy concepts.
3. Testing Company Relationship Predictions
What It Is: DNA testing companies like AncestryDNA, 23andMe, MyHeritage, and FamilyTreeDNA provide their own relationship predictions based on shared cM.
How It Works: Each company uses algorithms to analyze shared DNA and predict relationships (e.g., “1st cousin,” “2nd cousin,” etc.).
Strengths: Integrated into the test results, so no additional tools are needed.
Limitations: Predictions can vary between companies, and they may not account for complex family structures (e.g., endogamy or polygamy).
4. Theories of Relativity (AncestryDNA)
What It Is: A feature in AncestryDNA that uses shared cM and family trees to suggest how you might be related to a match.
How It Works: It compares your family tree with your match’s tree (if available) and identifies common ancestors or shared surnames.
Strengths: Combines DNA and genealogical data for more accurate predictions.
Limitations: Requires both parties to have linked family trees, and trees must be accurate.
5. Probability Calculators
What It Is: Tools like the DNA-Sci Calculator or SegcM | DNA Science use statistical models to calculate the probability of specific relationships based on shared cM.
How It Works: You input the shared cM value, and the calculator provides probabilities for different relationships (e.g., 80% chance of being first cousins, 20% chance of being half-siblings).
Where to Find It: Available online or as standalone tools.
Strengths: Provides precise probabilities for relationships.
Limitations: Requires accurate input and may not account for complex family dynamics.
6. Chromosome Browsers
What It Is: A tool provided by DNA testing companies (e.g., MyHeritage, FamilyTreeDNA, 23andMe) that visualizes shared DNA segments across chromosomes.
How It Works: You compare your DNA segments with a match to see where you share DNA. This can help confirm relationships or identify common ancestors.
Strengths: Provides detailed segment data, which can be useful for triangulating matches.
Limitations: Requires some expertise to interpret segment data effectively.
7. Triangulation
What It Is: A method used to confirm relationships by identifying shared DNA segments among three or more people.
How It Works: If three people share the same DNA segment, it is likely they inherited it from a common ancestor.
Strengths: Helps confirm relationships and identify common ancestors.
Limitations: Requires multiple matches and access to segment data (not all companies provide this).
8. GEDmatch Tools
What It Is: GEDmatch is a third-party platform that allows users to upload DNA data from different testing companies and compare matches.
How It Works: GEDmatch offers tools like the “Are your parents related?” feature, One-to-One Comparisons, and Tier 1 tools (e.g., triangulation and segment analysis).
Where to Find It: gedmatch.com
Strengths: Works across multiple testing companies and provides advanced tools for analysis.
Limitations: Requires uploading DNA data to a third-party site, which raises privacy concerns for some users.
9. Cluster Analysis (e.g., Genetic Affairs)
What It Is: A tool that groups DNA matches into clusters based on shared DNA and shared matches.
How It Works: By analyzing shared matches, it identifies groups of people who likely share a common ancestor.
Where to Find It: Available through Genetic Affairs or as an AutoCluster tool on GEDmatch.
Strengths: Helps identify branches of your family tree and potential common ancestors.
Limitations: Requires a large number of matches and some interpretation.
10. Manual Analysis (Spreadsheets and Trees)
What It Is: A DIY approach where you create spreadsheets or family trees to track shared cM values and relationships.
How It Works: You manually input data from your DNA matches and compare it to known relationships in your family tree.
Strengths: Fully customizable and can account for unique family structures.
Limitations: Time-consuming and requires a good understanding of genetic genealogy.
Choosing the Right Tool
The best tool or system depends on your goals and the complexity of your DNA matches:
- For quick estimates, use the Shared cM Project Chart or DNA Painter.
- For detailed analysis, use chromosome browsers, triangulation, or GEDmatch.
- For combining DNA and genealogical data, use Theories of Relativity or cluster analysis.