DNA Inheritance and the Power of Matching


When a commercial for a DNA kit airs, it all sounds intriguing and easy. Then, when the customer receives their results, and starts doing Google searches about how to interpret DNA, it can all become overwhelming. Genetic genealogists use terms like autosomal, shared cMs segments (centiMorgans), and other scientific-sounding terminology like SNPs and STRs. These terms need not be a stumbling block. Not only can the terminology be learned, but a lot of DNA analysis can be done without having to fully understand these foreign sounding terms.

Intro and inheritance

The DNA tests popular today were discussed in Price Genealogy’s article, Comparing the offerings of the five big DNA testing companies. These five tests provide “cousin matching,” a feature that lists other customers whose DNA matches yours at a certain threshold. The greatest accuracy is with close family connections within about six generations. The gradual tapering off of the usefulness of these matching cousins can be understood by the science of DNA inheritance. A 4x great-grandchild has only 1.6% of their DNA from one of their 4x great-grandparents. This is because each person gets half their DNA from each parent. We each have 23 pairs of chromosomes. When a child is conceived, only half of each parent’s DNA is inherited. Even a sibling will have different DNA because he or she ends up with a different 50% from each parent. This effect is the reason that relationships more distant than approximately 3rd to 5th cousins cannot be predicted from DNA with as much confidence. We may not share any identifiably unique DNA with relatives who are more distant.

Uses of matching

Before taking one of these tests, the customer should be warned that unexpected close family members may be discovered. Many customers have reported discovering that their own parents were not their biological parents. Although the accuracy of predicted relationships with these DNA tests tapers off the further out in the pedigree one goes, the estimates are pretty certain to about the second or even third cousin level. If you share an amount of DNA with a person that one of these relationships would predict, you will be related in one of a few possible ways—guaranteed. For example, if you take the test twice from one firm, or if you have an identical twin, the amount of shared DNA will fall into the same category. There are two possible relationships. The match is yourself or the match is an identical twin. It is indisputable. If a match is labeled as parent/child, then that person is either your parent or your child. The next category has a few more possibilities. Half-siblings, aunts, uncles, nieces, nephew, a grandparent or a grandchild all share about the same amount of DNA. Often one or two of these options can be ruled out when considering ages. Charts of possible relationships based on the amount of DNA shared are available at Ancestry.com.[1] Blaine Bettinger and Jonny Perl have an excellent interactive tool on the DNA Painter website to determine the probability of a DNA relationship based on the cM value. It is version 4 of his Shared cM Project 3.0 tool. Put a cM number in the “Filter” field and watch how the relationship chart changes. He credits The DNA Geeks for his stats. There are a lot of resources coming out to help us better utilize our DNA data.

The accuracy of these DNA tests for close relatives makes this testing especially helpful for those seeking close biological family, such as for an adopted person or a parent who put their child up for adoption. Researchers may also use these tests to determine the biological relationships of an ancestor (within a few generations) who was known or believed to have been adopted. Even if no such situation is believed to have occurred within a person’s family, those who take the test often match some unexpected close relative. There are other people out there in the world with these adoption or paternity mysteries in their families. For example, a great-uncle of mine had a child we never knew about. We do not even know if he knew about the child or not. After receiving a message from a relative match, I was able to resolve the connection through analysis of the DNA and research in historic records.

Even without these exciting television-worthy stories of solving adoption mysteries, there is genealogical value in matching. For example, those who are unaware of one or more of their 3x great grandparents can analyze their DNA matches to potentially determine their previously unknown ancestor. In my case, I like to research downward to determine my cousins who descend from the same 3x or 4x great-grandparents. By using traditional investigative methods I was able to identify many such cousins. Even after all that work, analysis of my DNA matches helped me identify other previously unknown cousins. In the process of researching their exact relationship, I was able to fill in my tree with information such as what happened to certain siblings of my ancestors, who they married, and the children they had.

Features and tools

The usefulness of matching can be further unlocked when using the various features and analysis tools available. Each of the five companies tends to have similar features, but there are also unique user experiences. A feature most of them have is the ability to check matches-in-common. This allows the customer to see common matches with any of their matches, whether that match’s relationship is already known or not. One way this is incredibly useful is to check your shared matches with close relatives who have no tree online, and whose relationship to you is unknown. If that unknown match appears in a list of your mother’s common matches, then you have determined that the match must be a relative through that side. Depending how many of your cousins and relatives have tested on various lines, it may be possible to determine from this list the precise line where a match must be related to you.

Another popular feature is Ancestry’s DNA Circles. These Circles automatically compile groups of customers who share DNA, when there is also a shared ancestor listed in some of their trees. Ancestry then suggests these to users, so even those without that ancestor in their tree have this hint about someone who might be their ancestor—because they share DNA with this whole group of other customers.

The built-in features of these five big companies are often enough to resolve many family history mysteries. If someone who has taken a DNA test wishes to enjoy more features and tools beyond what is available to them from the DNA testing company itself, it is a simple process to export their DNA data from the original website, and then import it into other third party websites or tools. These are broken into two categories: third party matching, and third party analysis tools.

Third party matching, tools, and analysis

The International Society of Generic Genealogy Wiki has a great deal of information on all the following topics for those who wish to go into more depth.[2] Learning resources will also be included at the end of this article. There are different types of DNA analyses. Some commonly referred to types include Chromosome Mapping, Triangulation, and Visual Phasing.

Some websites like GEDmatch and DNA.land allow users to find DNA matches with people who tested at other companies. Test takers only have to upload their raw data. These sites also include some analytical tools. There are a lot of third party tools available to analyze DNA cousin matches. One tool that many websites include is called the Chromosome Browser. This tool allows a person who has taken a DNA test to visualize portions of their DNA they share with one or more of their other matches. It is possible to learn which pieces came from which ancestor. Angie Bush has done a simple explanation of this on DearMYRTLE’S YouTube channel.[3] Chromosome Mapping is a type of DNA analysis that focuses on this sort of visualization that is possible with a Chromosome Browser. However, instead of simply visualizing the data, the idea is to tentatively map certain shared segments between two people who have taken a DNA test, thereby identifying those specific shared segments as belonging to a particular ancestor. This may sound like what Ancestry.com’s DNA Circles feature attempts to do automatically. In both cases, the researcher must remember that designations of specific DNA to a particular ancestor is tentative. Blaine Bettinger has explained this well in the first five minutes of his video on DNAPainter (one third party tool that assists users with Chromosome Mapping).[4] Visual Phasing has a similar purpose, to identify the specific DNA assignable to parents and grandparents, based on the autosomal DNA of at least three siblings. Even Triangulation is a similar kind of analysis in that it looks at a few people whose DNA matches at a specific location on the chromosome, and tries to identify the common ancestor from whom these people received this segment. For triangulation to be proved, all three test takers have to match each other on that same segment.

For those wishing to really dive into these topics, there are numerous useful learning resources. One educational subscription website is DNA Central by Blaine Bettinger.[5] DNA courses are popular at genealogical conferences and institutes, so that any large genealogical conference will likely have presentations on the topic. Webinars and YouTube videos are plentiful. Useful websites or channels include Legacy Family Tree Webinars,[6] Family History Fanatics,[7] and the Institute for Genetic Genealogy.[8] There are numerous excellent DNA blogs, including Roberta Estes’ DNAeXplained, Judy Russell’s The Legal Genealogist, Blaine Bettinger’s The Genetic Genealogist, Kitty Cooper’s Blog, and CeCe Moore’s The DNA Detectives. There are DNA testing company blogs and training videos as well.

Even with all the resources available, many people will find it cost effective to hire an experienced genealogist to help with their DNA and family tree mysteries. Please inquire with Price Genealogy to learn what your DNA can teach you about yourself and your ancestors, and for your broader genealogical needs.


[1] https://support.ancestry.com/s/article/DNA-Match-Relationships

[2] International Society of Generic Genealogy Wiki, “Autosomal DNA tools,” (https://isogg.org/wiki/Autosomal_DNA_tools : accessed 17 January 2019).

[3] Angie Bush on DearMYRTLE, “FTDNA Chromosome Browser,” YouTube (http://youtu.be/mMi0LfnLCns : accessed 17 January 2019).

[4] Blaine Bettinger, “DNAPainter,” YouTube (https://youtu.be/wyjcJxywTZI : accessed 17 January 2019).

[5] DNA Central by Blaine Bettinger (https://dna-central.com/ : accessed 17 January 2019).

[6] Legacy Family Tree Webinars, “DNA” (https://familytreewebinars.com/dna : accessed 17 January 2019).

[7] Family History Fanatics, YouTube channel (https://www.youtube.com/channel/UCmkKepHoafldMRZ4UNt0Akw : accessed 17 January 2019).

[8] Institute for Genetic Genealogy (www.i4gg.org : accessed 25 January 2019). Past videos are available for purchase.