Introduction To Genetic Genealogy

     Good Day Everyone. How is everyone doing? In this document, I wanted to provide an introduction to DNA or Genetic Genealogy. Some people whom take the various DNA tests are confused with their DNA results. What do these DNA results mean? This document serves to remove the confusion and to make things easier to comprehend. As we may already know, the concept of DNA is very powerful. Traits and features are passed from parents to children through DNA. We hear about DNA in the news, in paternity tests, exonerating falsely accused people, revealing human origins, etc. It shouldn't come as any surprise that DNA has now made it into the genealogy arena.  Let's now begin our discussion.


Basic Concepts

    There are two basic concepts that form the foundation of Genetic Genealogy.

A) When two or more living things share segments of DNA, there is or was, a shared common ancestor that lived in the past that connects those living things. For example, if you compare your DNA to your 1st cousin's DNA, then some of your DNA is going to be similar to your 1st cousin's DNA. This is because you and your 1st cousin have the same grandparents. The grandparents are the common ancestors. Each grandparent passed a percentage of his or her DNA all the way down to the 1st cousins.

B) The more DNA you share with someone, the more closer you are to that person. This means that your last common ancestor lived in a more recent time frame. For example, a brother and sister are more closer to each other, than either one of them are to their cousin. The last common ancestor shared between siblings are their parents, whereas the last common ancestor shared by 1st cousins are their grandparents. Siblings share on average, 30 to 50 percent of their DNA with each other. Cousins share 12.5 percent of their DNA with each other.


 When you get your DNA results, what the results mean depends on what type of DNA test you took. Let's now focus on the tests.

Types Of DNA Tests

    There are three basic types of DNA tests on the market 

1) Autosomal DNA Test - Humans have 46 chromosomes. The first 44 chromosomes are called the autosomes. An autosomal DNA test identifies shared segments of DNA across the first 44 chromosomes  When you and another person share a significant amount of DNA segments across your autosomal chromosomes, then you and that person have a common ancestor in your respective pasts. It is from that ancestor that you and your match received the shared DNA segments. The DNA amount, size, and segment length is represented as a unit of measurement known as the centiMorgan (cM). 

    An autosomal DNA test uncovers and reveals relatives (matches) on both sides of your family in a genealogical time frame. There are two major limitations that are associated with an autosomal DNA test. Let's take a look.

   The first limitation with an autosomal DNA test is that it cannot identify which side of your family a match is on. This is due to the fact, that it's impossible to know which chromosome you received from which parent. Of the 44 chromosomes analyzed by the test, 22 of the chromosomes you received from your mother, and the other 22 you received from your father. In order to know, you must test a parent, grandparent, or another ancestor or relative. If that ancestor or relative matched to the same person that you matched to, then you know which side of your family your match is on.

   The second limitation is that an autosomal DNA test can only go back 6 to 7 generations (last 200 to 300 hundred years). This is due to a natural process known as recombination which will be discussed in a separate document. An autosomal DNA test can identify parents, cousins, aunts/uncles, siblings, distant cousins, etc. An autosomal DNA test can also identify half relatives as well. 23andMe's autosomal DNA test is called DNA Relatives. Family Tree DNA's autosomal DNA test is called Family Finder. Ancestry.com's autosomal DNA test is called AncestryDNA. 

2) Y- DNA Test - In humans, chromosome numbers 45 and 46 are the sex chromosomes. Women have two XX chromosomes. A male has an X and Y chromosome. A Y-DNA test is strictly for men. The 46th chromosome in men is the Y- chromosome. As before, when two men share an identical amount of DNA on their Y-chromosomes, then those two men have a shared paternal (male) common ancestor. The Y-chromosome's inheritance is son -> father -> father's dad -> father's dad's dad, etc. Because your last name or surname (Williams, Jones, etc) is inherited in a similar fashion, a Y- DNA test can be used to see if a group of say, male Williams, are related. Given the fact that some surnames are fairly common (for example Williams, Jackson, etc), a Y-DNA test can help tremendously. On the other hand, a Y-DNA test can go back hundreds to thousands of generations in the past. This is due to the fact that the effects of DNA recombination are absent from a Y- DNA test. However, a Y- DNA test is mainly used for recent ancestry (last 200 to 300 years). In addition, a Y-DNA test gives you matches AND a Y-DNA haplogroup. (Haplogroups are explained in a separate document.) Family Tree DNA offers a Y-DNA test. Neither Ancestry.com nor 23andME offers a Y-DNA test. However both 23andMe and Ancestry.com do assign you a Y-DNA haplogroup. (For example E1B1A7A is a  Y-DNA haplogroup)

3) mtDNA Test - Inside a human cell, there is a structure called the mitochondrian. This structure is the battery of the cell. There are multiple copies of the mitochondrian inside a single human cell. Inside a single mitochondrian is a round circular piece of DNA called the mtDNA. The important thing to understand is that only women pass along their mtDNA to their children. A human male does not pass along his mtDNA to his children. Therefore, the inheritance of the mtDNA is child -> mother -> mother's mom -> mother's mom's mom -> etc. The same principle applies as already mentioned. When you and a person share an identical amount of DNA on your respective mtDNA's, then you and that person have a common maternal ancestor. The mtDNA changes very very slowly. Because of this, the mtDNA is mainly used for deep distant ancestry. 

     In other words, the last common maternal mtDNA ancestor that's shared between two or more people, may have lived thousands of years ago. Unless your entire mtDNA is identical to someone else, then you and that person are very likely distantly related. The effects of DNA recombination are absent from a mtDNA test. This allows a mtDNA test to go back and span many generations (years) in the past.

   A mtDNA test gives you matches AND a mtDNA haplogroup. (Haplogroups are explained in a separate document.) Family Tree DNA offers a mtDNA test. Neither 23andME nor Ancestry.com offers a mtDNA test. However, 23andMe does assign you a mtDNA haplogroup. (For example, L3c2a is a mtDNA haplogroup).

4) BGA (Admixture) DNA Test - There is a 4th separate DNA test called a BGA or Admixture DNA Test. BGA stands for biogeographical analysis. A BGA test attempts to use your DNA to identify what part of the world your ancestors originated from. If your DNA test results have something like - 69% African, 21% European, and 10% Asia, then you had a BGA test performed on your autosomal chromosomes.

   BGA Tests are controverisal. Currently the accuracy and validity of such tests are questioned and generally not accepted by the scientific community. Therefore you should accept BGA tests with a grain of salt. Such tests are not conclusive and can change with the introduction of new data. In addition, the results of such BGA tests can vary between different DNA testing companies. This is mainly due to the use of different algorithms and testing methods used by each company. The same holds true for any online 3rd party tool such as Gedmatch, Dodecad Ancestry Project, etc that claims to produce BGA results.  BGA tests will be fully explained in a separate document.

Summary:  In short, a Y-DNA test looks at only one side of your family (father's strict paternal side). A mtDNA test looks at only one side of your family (mother's strict maternal side). An autosomal DNA test looks at both sides of your family (both meaning everything. For example your dad's mom side or your mom's dad side. This can include the father's strict paternal side or the mother's strict maternal side as well).

Hope that helps everyone!!!!  Please let me know if you have any questions.

Thanks
Steve Handy