How Accurate is My List of Genetic Relatives?
| DNA Wiki Topics | |
 | |
| DNA Companies | |
| Public DNA Databases | |
| Classes | |
| DNA Methodology | |
| |
| Scientific Details | |
| |
| Ethics | |
Your list of genetic relatives is extremely accurate and is much more valuable to a genetic genealogist than your ethnicity. Ethnicity results infer ethnicity by comparing against reference populations and statistical models. Genetic relatives is determined, however, by comparing against DNA matches in the database directly.
When you take a DNA test a raw data file is produced and uploaded into the company’s database. This file is compared against every other test in the database. When enough SNPs in a row match between your raw data and another person’s it forms a matching segment. The longer the segment and the more segments there are, the closer the relationship must be. Both test takers inherited the matching segments from the common ancestor.
False positives only happen for the smallest of matches and are statistically impossible for segments larger than 15 cM’s. If you have a DNA match that matches you but neither of your parents, it is likely a false positive and a distant match.
| Segment size in cM | Odds of Being a False Positive |
| 3 | 97% |
| 5 | 48% |
| 7 | 10% |
| 15+ | 0% |
Reducing False Positives Using GEDMatch
In GEDmatch, the Phasing tool can help you filter out most if not all of the false positives. This will allow you to focus only on the small matches that are likely to be really. To use this tool you must upload your DNA and one or ideally both of your parent's DNA as well. The phasing compares your DNA to your parent(s) and creates two new kits, one representing only your paternal DNA and one only representing your maternal DNA. The new kits will have the same kit number as your original except they will end in "P1" and "M1" or "NP1" and "NM1." Compare the mystery relative to both kits. If they match one of them they are a real match. If they match neither then the match is false.
If you do not have access to the DNA of a parent you can do a similar thing with one of your own children. First upload your DNA and your child's DNA to GEDmatch. Run the Phasing tool on your DNA and theirs to create paternal and maternal kits for your child. Second use the evil twin tool on yourself and your child. The Phasing tool will create a kit with all the DNA you passed on to your child and will end in "M1", "NM1", "P1", or "NP1" (depending on your gender). The Evil Twin tool will create a kit with all the DNA you did not pass on to your child and will end in "M2", "NM2", "P2", or "NP2" (depending on your gender). Between the two kits all of your DNA will be represented. Compare the mystery relative to both kits. If they match one of them they are a real match. If they match neither, then the match is false.