Plain language introduction to DNA for genealogy

DNA is found in the 23 chromosome pairs that include the sex chromosome (XX for females, XY for males). DNA is also found in mitochondria, which is the “powerhouse” of cells, and generate most of the cell’s supply of energy. Due to the way DNA is inherited, it can be used for genealogy, helping you to locate close or distant relations, as well as allowing you to trace a much deeper ancestry. There are several different ways DNA is used for genealogical purposes.

Autosomal DNA

Autosomal DNA is sometimes referred to as atDNA or auDNA. Currently, this the most popular form of DNA testing for genealogy is autosomal DNA testing. This tests parts of the first 22 chromosome pairs, and excludes the sex chromosome pair (XX or XY). The aim of testing is to sample parts of DNA that is not subject to change over recent generations, allowing you to compare yourself with people who share the same DNA sequences. When you share a certain amount of DNA (generally given in centimorgans and abbreviated as cM) with another person, you are considered to be a match.

The limitation of autosomal DNA testing is that it is only useful for finding matches within about 5 generations (to about the 5th cousin level). Beyond that, it is difficult to prove that a relationship is not just random chance, as the shared segments of DNA are so small. This is because you inherit about 50% of your DNA from your mother and the other 50% from your father. Therefore, you have about 25% of the DNA of each of your four grandparents. The mix that you inherit from your parents is random, so you will only overlap to a certain degree with a brother or sister.

Here is a real-life example of matching amounts of autosomal DNA between family members. This is how I actually match with my parents and maternal uncle.

Autosomal DNA X DNA
Relationship Total auDNA (cM) Largest segment (cM) Generation Total DNA (cM) Largest segment (cM)
Father 3587.1 281.5 1 0 0
Mother 3584.5 281.5 1 196 196
Uncle 1804.1 159.5 1.5 61.4 53.1

Some companies test the X-DNA, so I’ve included the information here. You can see that I have a much higher amount of X-DNA matching my mother than I do my uncle.

Autosomal DNA is also used to determine ethnic admixture, which is a percentage of DNA that your share with various ethnic groups. The various testing companies and analysis services have developed a number of different methods of reporting ethnicity. It is all fairly arbitrary and I haven’t personally found this particularly useful. It shows that I am 100% European, and have ancestors from the United Kingdom and a number of nearby countries. There are no surprises there.

X-DNA

The X chromosome is one of the two chromosomes that determine the sex of a person. Women have two X chromosomes. They inherit one X chromosome from their mother, and one from their father. The X chromosome from their father is an exact copy, while the one from their mother is a combination of her two X chromosomes. Men have one X chromosome, which they inherit from the mother, and is also a combination of her two X chromosomes.

X DNA
Relationship Total DNA (cM) Largest segment (cM)
Mother 196 196
Uncle 61.4 53.1

Looking at the real-life example again, I get 100% of my DNA from my mother, which is 196 centimorgans. This is a mixture of my mother’s mother’s and father’s X-chromosome DNA. As you can see, my uncle, who only has his mother’s DNA, shares only 61.4 centimorgans.

When using X-DNA to find help find a common ancestor with someone who shares both autosomal DNA and X-DNA, there are two important things to understand:

  • Any father-to-son relationship can be excluded, as a boy only gets one X chromosome from his mother.
  • Because a girl inherits an exact copy of her father’s X chromosome, strong X-DNA matches are more likely to share a common ancestor along a father-daughter branch of the family tree.

For in depth information, see: X Chromosome Recombination’s Impact on DNA Genealogy.

Y-DNA

The Y chromosome is what determines that a persons sex is male. A man inherits the Y chromosome directly from his father. As such, only men can get Y-DNA testing. Y-DNA mutates at a fairly predictable rate, so it is possible to estimate how many generations back the common ancestor was. Most importantly, it is 100% certain that they are male and on your father’s, father’s father’s line (paternal line). Y-DNA testing is popular for surname projects and for those trying to find a geographic place that their paternal line lived.

The are parts two Y-DNA testing. The first is testing of short tandem repeat markers (STRs). The more Y-STR markers tested, the higher the accuracy of matching relatives and estimating how many generations separate them and a common ancestor. It is generally recommended that 37 markers is the lowest that you should test if you want accuracy. However, on the various Family Tree DNA projects that I’ve joined, the recommendation is at least 67 markers, and preferably 111 markers. By testing these markers, you can be placed on the Y-DNA family tree and discover your paternal ancestors’ migration patterns, which go back thousands of years. Your overall group is known as haplogroup.

For more precise matching with other men sharing Y-DNA, the second type of testing is single-nucleotide polymorphisms (SNPs). These are usually tested for based on STR test results. These will find the sub-group that you belong to. This sub-group is known as a subclade. Newer tests, known as next generation Y-DNA tests (e.g., Family Tree DNA’s Big Y test and Full Genome’s Y Elite 2.1) go a lot further and discover both known and previously unknown Y-DNA SNPs. When a sufficient number of men have matching SNPs, the Y-DNA tree grows and new branches are added. It is common now for the location on the Y-DNA tree to be given as the terminal SNP, so R for the main branch and M269 as the terminal SNP that defines its placement and written as R-M269.

The limitation is that this is always on your father’s direct paternal line. It will not tell you anything about his mother’s father.

My father’s Y-DNA haplogroup is R, as is mine. This was determined by the taking the Y-DNA STR tests (Y-DNA 111 markers). We belongs to the major branch is R-M343 (R1b) and, more specifically, R-M269 (R1b1a2), which is the most common branch in Western Europe (Western Atlantic Modal Haplogroup), shared by an estimated 2.62% of males.

Taking Family Tree DNA’s Big Y test, we can now trace the next major branch in the Y-Tree to R-P312 (R1b1a1a2a1a2)From there, it branches down R-L21 (making up between 25-50% of the male population in England and Ireland),  R-DF21 (this branch is estimated to be about 3000 years old), and down to R-Z29559. According to The Big Tree, the Y-DNA branching ends up in a group with only 2 other men. Unfortunately, neither of them have the family name Wilson, so it may be that they are related hundreds of years ago.

At the time of writing, I have ordered an SNP test from YSEQ to see if I match R-Z29559. In the future, I will take the Full Genomes Y Elite 2.1 test and use it as a comparison with my father’s Big Y test.

Mitochondrial DNA

Mitochondrial DNA is often referred to as mtDNA. While Y-DNA is only passed down from father to son, mtDNA is passed down from the mother to either a son or daughter. Similar to Y-DNA, mtDNA can be used to trace your maternal ancestors’ migration patterns, which go back thousands of years. Your mtDNA is classified into a haplogroup, and the more detail you test in (mtDNA full sequence, compared to only testing HVR1 and HVR2 coding regions) will give you more details on where your maternal ancestors came from.

The limitation is that this is always on your mother’s direct maternal line. It will not tell you anything about his father’s mother.

My father’s mitochondrial DNA haplogroup is U4. Initially, we only tested the HVR1 and HVR2 coding regions. By doing the mtDNA full sequence test, it showed that his haplogroup is actually U4a1a. Having joined the U4 Project group on Family Tree DNA, this was further refined to U4a1a1. The U4 haplogroup originated about 25,000 years ago and seems to have been more common in Northeast Europe. U4a1a1 was found in Poland and Russia.

Conclusion

For any type of DNA testing, the biggest limitation is whether or not any close relatives (within several generations) have tested with the company you used. As it is believed that we all have a distant common male ancestor (“Adam”) and a distant common female ancestor (“Eve”), we are all distantly related. Therefore, it makes sense that we would want to find closer relatives. When it comes to autosomal DNA testing, the advice seems to be as follows:

  • If you live in USA, then definitely test with Ancestry DNA. They have over 2 million people who have tested, and mostly from the USA.
  • If you live in Europe, Family Tree DNA might be a better option, and seems to have been more popular with people living outside of the USA.
  • If you test with either Ancestry DNA or 23 and Me, you can download your results and upload them into Family Tree DNA. For a small fee, you can use the searching to find all your matches [At the time of writing, the functionality is currently unavailable, but rumour has it that this process will be working again soon, maybe by early 2017].
  • The best advice is to test with all 3 of the major services (or at least Ancestry DNA or 23 and Me, then upload the results to Family Tree DNA and pay for the upgrade).

Whichever service you choose, you can download your results and upload them to GEDmatch. This is free, and allows you to search for matches with people who tested with other services. However, most people won’t have uploaded the DNA results to GEDmatch.

Another good option is to upload it to DNA.land. This is a new service, so there are not many users, and therefore few matches.

When it comes to Y-DNA and mtDNA testing, there are fewer options. Family Tree DNA seems to be the most popular choice, with YSEQ being very useful for some targeted tests that are unavailable elsewhere.

For “next generation” testing of Y-DNA, then Big Y (Family Tree DNA) and Y Elite (Full Genomes) are currently the only options.

 

 

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