Getting back your DNA test results but faced with a whole bunch of information that you’re confused about? Join us in understanding how exactly are you related to your DNA matches.
Well, now that you’ve taken a DNA test, what exactly do your DNA matches mean? How are you related to your DNA matches?
DNA matches are people who share specific segments of their genetic composition with you. When you receive your DNA matches, the first instance of confusion probably will arise when you see the term centiMorgan, besides the fact that you might not recognise the names of most of your matches. Through understanding the fundamentals of what centiMorgan are and their function in measuring your DNA will definitely help you make sense of your DNA results.
Breaking down centiMorgan
When you first see the abbreviation cM, the first thing that pops to mind would probably be linked to centimeters or inches. Well, in this case, this is completely different from the cm or centimetres that most of us are familiar with.
Centimorgan, abbreviated as cM, is a unit of measurement found in your DNA. A centiMorgan is used to describe the amount of DNA and the length of specific DNA segments that is shared with your relatives. CentiMorgan as a measurement of probability provides an indication on whether the segment of DNA being analysed is exactly what it was a generation ago. A 40cM segment means there is a 40% chance that a crossover event happened. It can be used to roughly measure the number of generations between two individuals (based on their shared DNA). The total length of all your chromosomes is approximately 7400 cM.
The higher the number of shared centiMorgan you have with someone, the more closely related you both are.
DNA matches by AncestryDNA, 23andMe, MyHeritage, FamilyTreeDNA and Living DNA are all measured using centiMorgan (cM) however the same DNA match can have different centiMorgan at AncestryDNA, 23andMe, MyHeritage than FamilyTreeDNA. This is because FamilyTreeDNA uses a different threshold for segments and includes them down to 1 cM.
How do I find my Shared DNA?
A DNA testing company will provide you with your DNA matches after analyzing your DNA. The DNA markers (called SNP's) are tested against one another and compared to determine how long an individual has at least one DNA letter match (A, C, G and T). For each chromosome, we inherit two strands (one from each parent), so we have two letters for each marker. As an example, "AA" and "CC" is not a match, but "AA" and "AC" is.
Every DNA testing company has its own threshold and additional rules regarding how long a DNA match segment is. When the DNA segment is 7 cM or longer, it usually is recognized as valid, or IBD, and shown as a shared segment.
Valid in the sense that the common ancestor who gave this ancestral DNA segment to its descendants has lived up to 500 years ago. This time frame was chosen to match the genealogical time frame as the oldest church books started in the 16th century. If you want to identify common ancestors further back in time you have to lower that threshold. A 6 cM threshold identifies common ancestors from 1200 AD onwards (Source: Prof. Itsik Pe’er - “The Genome of Netherlands”).
Based on the prediction of your relationship with your DNA matches, these matches will be divided into different categories such as Parent/Child, Grandparents/Grandchildren, Full Sibling, Half Sibling and more. This chart below provides an estimate using the number of centiMorgan to help you determine your relationship with your matches.
As you can see, there are many similar ranges of shared cM that overlap between the different categories. That being said, while understanding how a shared DNA % or centiMorgan can explain your relationship with your matches, remember that it is just a prediction and it cannot always determine your relation to a person. This is especially true for endogamous groups where DNA matches are usually related in more than one way.
Brit Nicholson Tool for Shared cM
The Brit ciM tool is a genetic genealogy tool that calculates the “most probable relationship types for your DNA matches”. Using shared DNA data, this tool will predict the estimate of the relationship category that your DNA matches most likely falls into.
The Brit ciM tool is the first ever prediction tool that considers the minor differences between maternal and paternal relationships, and showcases the key differences that can occur between each relationship category.
This is the screenshot of the Brit ciM tool from the main page.
Referring to your DNA test results, enter the shared cM or percentage that was given to you into the appropriate box. The options only include DNA tests from AncestryDNA and 23andMe as they provide the most accurate data.
This is an example of how to enter your DNA data into the tool.
After you have clicked submit, you will see the calculated probabilities of how your DNA match could be related to you. Referencing to the sample below, the probability of a shared cM of 2,345 has the highest probability (91.4%) of the person being your full-sibling.
Example of the estimated probability that is generated using the Brit ciM tool.
Interpreting your shared DNA/cM can be complicated especially when you’re new to DNA testing, but we hope that this article helps clear things up!
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