First and foremost, all genealogical sources, including DNA testing, have the potential to reveal unexpected relationships or the lack of an expected relationship. You, the tester, must anticipate these possibilities and consider the potential emotions and reactions of ALL involved. Be prepared for the unexpected. The worst possible unexpected result is finding out a parent is not your biological parent or who you thought was a full sibling is only a half sibling. If this knowledge is going to rip your life, or the life of your parents & siblings apart, it may be best not to DNA test. On the other hand, you have an absolute right to your own genetic heritage. Each individual must make the best decision for themselves.
If you decide to Autosomal DNA test, there are some basics that you must understand in order to interpret your DNA results.
1) Family relationships
It is imperative to understand family relationships and where that relationship fits in a family tree
Full siblings share parents (common ancestor couple)
First cousins share grandparents (common ancestor couple)
Second cousins share great grandparents (common ancestor couple)
Third cousins share 2nd great grandparents(common ancestor couple)
Fourth cousins share 3rd great grandparents (common ancestor couple)
Fifth cousins share 4th great grandparents (commn ancetor couple)
The word removed means that the cousins come from different generations
Once removed means that cousins are one generation apart
Twice removed means that cousins are two generations apart
Your first cousin once removed is the child or parent of your first cousin
Your second cousin once removed is the child or parent of your second cousin
Your first cousin twice removed is the grandchild or grandparent of your first cousin
Your second cousin twice removed is the grandchild or grandparent of your second cousin
Drawing out the relationship on paper is a good way to catch hold of the meaning of removed cousins. Here is what my own parent’s cousin relationship looks like and how I am related in multiple ways to Thomas Morris, who serves as my 2nd and 3rd great grandfathers.
2) How you got your DNA
At conception, every human inherits 6 billion base pairs of DNA packaged into 46 chromosomes
You get 3 billion base pairs (23 chromosomes) from Mom and 3 billion base pairs (23 chromosomes) from Dad
The first 22 chromosome pairs are called Autosomes and are numbered 1 through 22 (and is where the name Autosomal DNA testing comes from)
The 23rd pair are called the sex (or gender) chromosomes X and Y
Men inherit an X chromosome from their mother and a Y chromosome from their father (XY)
Women inherit an X chromosome from their father and an X chromosome from their mother (XX)
Your father got 50% of his DNA from his father and 50% from his mother = a combination of all four of his grandparents
Your mother got 50% of her DNA from her father and 50% from her mother = a combination of all four of her grandparents
You received 50% of your DNA from your father and 50% from your mother = a combination of all four of your grandparents
The DNA you received from your grandparents, through your parents, is a combination of all 8 of your great grandparents, so, every DNA match you have will lead from one of your grandparent's lines through your great grandparents
In reality, DNA testing traces all four of your grandparent lines
There are methods and tools (Leeds Method, Q&D Trees, GEDmatch, DNA Painter, Triangulation, Shared Matches, In Common With, Chromosome Browsers, cM Project) that can be used to assist in determining the relationship to a DNA match and which of your four grandparent lines a match connects to.
3) How does the DNA testing company know what relationship you share with a DNA match?
Using a measurement called centimorgans (cM) and a mathematical formula (called an algorithm), the DNA testing company can work out the genetic distance (relationship) between two people based on the amount of shared DNA. The chart below shows the relationship and the amount of DNA that relationship will share.
Note: 2nd cousins and closer will ALWAYS share some DNA. If you share no DNA with a known 2nd cousin, then you are not 2nd cousins.
About 10% of 3C will not share DNA
About 50% of 4C will not share DNA
Note: when DNA testing companies do chromosome comparisons, they can't distinguish between the two chromosomes in a pair (maternal and paternal). Instead, they treat them as one combined chromosome. This means that when you match someone on a segment, the DNA testing company can't be sure if the DNA was inherited from your mother (maternal side) or from your father (paternal side) - that’s what you have to figure out and there are tools and techniques that can be used to help you do this.
4) The language of Genetic Genealogy (DNA testing)
In order to understand your DNA results, you must understand the language. Here are the terms (and the meaning) most used that you will want to know
A measurement of genetic linkage that shows how close or far a relationship is between two people. The higher the cM value, the closer the relationship
Distinct segments of DNA on a chromosome that is compared between two people to see if they genetically match. The amount of matching SNPs is measured in cM
A section of SNPs. A ‘matching segment’ is a section that is identical between two people
Start Location and End Location
Individual markers (called base pairs - the things that SNPs are made of) within a chromosome. A segment of a chromosome can be identified by these location markers
IBS (Identical By State) - SNP values that match between two people due to race or ethnicity (example: Jewish & African)
Note: Matching segments smaller than 7 cM and 700 SNPs have a high likelihood of being IBS (matching by race or ethnicity) and should be ignored
IBD (Identical By Descent) – SNP values that match because they were inherited from a common ancestor
Most Recent Common Ancestor - the ancestor from which you and a DNA match received your shared DNA
5) Resources I use frequently
Books (available on Amazon for a cost and most likely at your local library for free):
Genetic Genealogy in Practice by: Blaine T. Bettinger & Debbie Parker Wayne
The Family Tree Guide to DNA Testing and Genetic Genealogy by: Blaine T. Bettinger
Websites & Blogs (Free unless noted):
The Genetic Genealogist https://thegeneticgenealogist.com
Kitty Cooper’s Blog http://blog.kittycooper.com
Your Genetic Genealogist http://www.yourgeneticgenealogist.com
DNA Detectives https://thednadetectives.com
Ancestry DNA https://support.ancestry.com/s/ancestrydna
Family Tree DNA https://www.familytreedna.com/learn
Wheaton Surname Resources https://sites.google.com/site/wheatonsurname
The Legal Genealogist https://www.legalgenealogist.com
International Society of Genetic Genealogy https://isogg.org
Genetic Genealogy Tips & Techniques https://www.facebook.com/groups/geneticgenealogytipsandtechniques/
DNA Central https://dna-central.com ($)
The Shared cM Project https://dnapainter.com/tools/sharedcmv4 (this will become one of the most important tools you use)
DNA Concepts for Genealogy: Y-DNA Testing Part 1 2 3