Simple & Accurate DNA Matching: Understanding the Process

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How accurate is DNA matching?

Almost every person who has sent their DNA sample for testing has questions around the accuracy of DNA matches reported by the genealogy companies. And why not? Getting to know your DNA testing results for the purposes of genetic genealogy is an important aspect in finding the right match to expand your family tree.

How does ancestral DNA matching work?

Once you get your uninterpreted DNA test results from the DNA testing services (23andMe, AncestryDNA, MyHeritage DNA, Family Tree DNA ) you can upload, interpret, and get DNA match lists from other external third party websites like DNA Land and Gedmatch (more about them below).

DNA matching involves comparing your DNA to every other person using a proprietary database. The match results will give you an outline of how closely you are related to your biological relatives such as parents, siblings and distant cousins. Every individual inherits a portion (~50%) of their parent’s DNA, which is inherited a generation earlier from their parents and the chain continues. The chain is connected with by a common ancestor. 

When your DNA is compared to another person, chances are that you share traces of DNA originating from this common ancestor. This is called ‘shared DNA’. There are several probabilities and statistical tests to ensure that your DNA match is indeed accurate. The shared DNA is measured in terms of genetic distance measured in centimorgans (cM). The higher this measure the more the match.

How does phasing impact accuracy of DNA analysis and matching?

Phasing is a technique that allows you to assign proportions of your DNA to your parental DNA by matching alleles (the As, Cs, Ts, and Gs). Phasing becomes extremely important in DNA matching because It helps in identifying what portion of your DNA matches with your maternal or paternal side. It enhances the accuracy rate when finding distant relatives such as cousin from your maternal or paternal side. If you are able to provide phased DNA (trio analysis), then you can also determine whether the DNA matches such as cousins come from your maternal or paternal side.

What parameters from your DNA profile are taken into consideration?

DNA markers (aka SNPs or Genetic markers) are genetic variations in alleles at specific locations in your chromosomes. These polymorphisms are used to measure the genetic distance. Different services adopt different methods to caclulate DNA matches. Broadly, the below parameters are used to find a potential match.

  • Rs # – An rs id or a SNP id is a unique number used by DNA testing services and other DNA databases to refer to specific SNPs. rs id stands for Reference SNP cluster-ID.
  • Chromosome – A chromosome is a single long chain thread-like structure that carries information in the form of genes.
  • Position – It signifies the variant position on the chromosome.
  • Genotype – A genotype is an individual’s allele information.

Shared DNA segments between two DNA samples

Shared DNA coordinates in centimorgans (cMs)

Are your DNA match results accurate?

The table below shows how AncestryDNA computes accuracy when matching DNA relatives. Depending on which company performed your DNA test, the parameters would change marginally. Most of these are just small variations as different companies set different thresholds. You can find more information on the thresholds used by different companies on ISOGG wiki

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How to improve your DNA profile relationships? 

If you are one of those folks who has found >100,000 fourth cousins, I am sure you are in a state of confusion. If you want to know if these are accurate, then please refer to this handy guide on what a fourth cousin is? Do remember that your chances of find an accurate match drastically improves if you:

  • use phased DNA from your family tree.
  • expand your DNA network by connecting with your matches and sharing your family history.
  • monitor your network and if possible map these relationships on a family tree.
  • come across DNA matches, ask them if they happen to have a family tree.
  • use different parameters like Surnames for instance to see which DNA profile matches are on your paternal or maternal side of your family.

Advantages of uncovering a DNA match

  • It tells you more about your identity.
  • Helps you connect and share your DNA to explore relationships.
  • Gives you options to explore your mtDNA and Y-DNA haplogroups besides just autosomal DNA matching.

Happy DNA matching. Please let us know your experience in the comment section below 🙂

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