Tests, Differences & The Prize

The Science of Your Direct Paternal Line

Your Y-Chromosome is a sex chromosome. Sex chromosomes carry the genetic code that makes each of us male or female. All people inherit two sex chromosomes. One comes from their mother and the other from their father. You and other men receive a Y-Chromosome from your father and an X-Chromosome from your mother. Men and only men inherit their father's Y-Chromosome. Thus, it follows the same path of inheritance as your direct paternal line. Paternal line DNA testing uses STR markers. STR markers are places where your genetic code has a variable number of repeated parts.[Source: FTDNA]

When thinking about DNA testing it is important to understand what test you are taking and for what reason. This is a troublesome detail that most of us skip over for years as we keep searching through the same old list of "matches" with disappointment. And, for many of us, even when we have a match provided to us by whatever testing company we used, we likely don't understand what the result means or what to do with next. This page will deal briefly with Y-DNA testing as an introduction mostly to explore the differences between Y-DNA testing and Autosomal (AtDNA) DNA testing.

When a male is tested with a Y-DNA test it is recommended that a test of 37, 67 or 111 markers be purchased (greater sized marker tests may be available over time). So, what are these markers we are testing? Wikipedia states a genetic marker may be a short DNA sequence, such as a sequence surrounding a single base-pair change (SNP). For our purposes this is called a short tandem repeat (STR) and occurs when a pattern of two or more nucleotides are repeated and the repeated sequences are directly adjacent to each other.

Well, what is being tested and reported? If you buy a 67 marker test, you have bought a defined testing protocol that knows 67 exact locations on a Y chromosome and in that exact location, how many times they would expect to see a short tandem repeat. The test result you receive is your personal number of short tandem repeats on that exact marker. As an example, the author's Y marker DYS393 has a value of 13 repeats; DYS390 has a value of 22 and so on for either 37, 67 or 111 of these exact markers. We test takers are mostly interested in finding related individuals so we are looking for others with similar STR results.

STR marker values change slowly from one generation to the next so testing gives us a distinctive result set which forms a signature for our paternal lineage. To determine the generation a common ancestor lived in with a match depends on the level of test you purchase. Because Y DNA is a forever kind of genetic sample a match may be recent, but it may also be hundreds of years in the past. Because of this, the test developers have developed confidence levels of various test types. This is about the same as making a bet in Las Vegas; I am betting that my common ancestor is alive in a specific generation. In my Y-DNA case, I tested 111 markers that resulted in me matching 109 STR's out of 111 STR's for a genetic distance of 111-109 of 2. The result for my bet placed was that I have a 95% chance that the common ancestor with my match and myself will be found living within 9 generations.

Autosomal DNA and testing

Autosomal DNA (AtDNA) is a term used in genetic genealogy to describe DNA which is inherited from the autosomal chromosomes. An autosome is any of the numbered chromosomes, as opposed to the sex chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes (the X chromosome and the Y chromosome). Autosomal DNA is inherited roughly 50% from each parent. It is randomly shuffled up in a process called recombination and the percentage of autosomal DNA is diluted with each new generation. Because autosomal DNA is a mixture of your mother’s and father’s DNA, it is unique to each person. Both men and women can complete this testing. Testing technology is currently limited to the point that an autosomal (Family Finder) test cannot distinguish between matches from your mother’s side versus your father’s side. And, the usual effective range of detection is between 5-8 generations.

With this kind of DNA testing you will hear the phrases DNA segments and Centimorgan (cM). At this point, remember that is what you are wanting to see, and, the bigger the DNA segment, the better. For now, just focus on segments bigger than 10 Centimorgan (cM). Don't worry about what that really means, it is just a size thing.

Generations are differently thought about comparing Y-DNA and AtDNA. With Y results we think of confidence levels of a genetic distance on a 67 marker test means a certain confidence of containing your common ancestor within so many generation. For AtDNA, you must review the chromosome details to determine if a match on a certain Chromosome, even if greater than 10cM, is a "shared segment match" or an "inherited segment match". Only inherited matches matter. If you see one segment which matches like this then you have to determine if this is a fluke match or if can be proven; you do that by finding a 3rd person with the same matching segment that you know something about such as a known cousin.

You can go through a process of setting aside those ancestors that don't apply to this specific match possibility. If your cousin is a cousin because they match through your GG grandparents on your paternal side, you can stop including your maternal ancestors because they don't apply. So, then the prospective new match would be in that common paternal ancestor's generation or older.

To review shared segments to determine if they are simply shared or if they are inherited matches we compare the details of the match in various ways. Each of us will find a methodology which suits them; most of us start with chromosome browsers then also review the start and stop metrics of the segment on each chromosome. Each of the testing companies have a certain set of tools to help you make your evaluations. They are certainly helpful up to a point then many of us use 3rd party tools to assist. These tools are listed in the resources section.

Something which is a question related to viewing chromosome browsers is if we have 46 Chromosome, why do you only see 23 chromosome shaped images? Remember, the test cannot distinguish between matches from your mother’s side versus your father’s side. So, the view you see is a representative composite of each of the two pairs of each respective chromosome. You will need to puzzle out by triangulation with others if a match segment is on the paternal or maternal chromosome.

Genetic Generations
Visualizing AtDNA Segmentation & Family Tree Volume


An autosomal DNA test works across genders and helps us find cousins with whom we share DNA from all parts of our family tree, paternal and maternal alike good for about 5-8 generations. While Y-DNA changes very little over long periods of time as it is passed from father to son, autosomal DNA changes with each and every generation through a process called recombination.

This is an almost unbelievable plot device that could be found in a Sci-Fi movie where all of the pieces of autosomal DNA we inherited from our parents gets mixed and jumbled before half of those pieces gets passed on to the next generation. This process happens independently for each child created by the parents, so even full blood siblings won't get exactly the same pieces from their parents.

AND, Mom and Dad were created by the same sort of random mixing of their parent's DNA. That's why we say that siblings share 50% of their DNA, and that they share 50% of their parent's DNA, but it won't be the same 50% from sibling to sibling because of the randomness of recombination. (This is as good as excuse as any for that weird Uncle or Aunt and the extraordinary differences some times between brothers and sisters.)

So how do we get these segments that we are searching for in our genetic genealogy quest? Wikipedia explains that in human cells the different chromosomes occupy separate area in the nucleus called "chromosome territories". This physical separation of different chromosomes is important for the ability of DNA to function as a stable repository for information, specifically, the information which you know as self. Chromosomal crossover is when two individual chromosomes break, swap a section and then rejoin. Recombination allows chromosomes to exchange genetic information and produces new combinations of chromosomes; these newly created segments are the segments we want to identify and track. To overly generalize, through a process, driven through the power of Meiosis the new combinations of chromosomes are made up of pieces of different sizes variably selected and passed downward from your personal pool of ancestors.

The image above [from PBS] Visualizing AtDNA Segmentation, is one of the best images found that is meant to convey several important thoughts to consider. Please note the row of ancestors listed as generation 4; these represent the eight contributing Great Grandparents of generation number 1 (self). Notice that each Great Parent consists of two sets of chromosomes of a distinct color; to illustrate the combining of chromosomes by Meiosis, select just one Great Grandparent and watch the assigned color downwards. You will note how segments of the ancestor DNA is represented in the generation 1 DNA.

In this select case, all the DNA segments shown would be considered IBD (inherited by descent). But of course, it is not that simple because real life biology intrudes with the fact that on average, in terms of DNA sequence, all humans are 99.5% similar to any other humans. The other 99.5% that you would match to is Shared DNA and is referred to as IBS (Identical by state). So, when it looks like you are a match with someone else, are you saying you are a match because you have 99.5% same DNA and therefore human? Or, are you saying you are a match because you have reviewed the detail and can confirm that your DNA segment has the same location, size and relative position of start and stop as one of your ancestors?

This is The Prize you are looking for and this is where the real work starts for us genetic genealogists, sorting through the details tracking down specific segments, triangulating with others all to prove the case that someone is a true genetic ancestor through your DNA received by IBD.

Autosomal DNA statistics

As you now know, autosomal DNA is inherited from both parents and it is randomly shuffled up in a process called recombination. This results in the percentage of autosomal DNA coming from each ancestor being diluted with each new generation. This dilution has the effect of capping the effective detection period of IBD matches to 5-8 generations.

This does not mean you have lost your ancestor, it simply means that their original DNA contribution amount has been recombined and diluted beyond a detectable range. This is a variable process and longer matches have been reported. The author has a 109/111 Y-DNA match with a genetic cousin yet the autosomal match is reduced to 3.5 cM's which is below what we would consider to be a valid match.

A statistical foundation has been laid to understand and predict relationships based on autosomal DNA matching. Data collection is ongoing and is an active field of study. The take away information for understanding is that recombination over generations has an effect on shared IBD dna; this understanding is important as you undertake match evaluation in spreadsheets, 3rd party programs, chromosome browsers and charts.

Your Paper Trail Family Tree

So, how do you describe to another interested person how you relate to your 9th generation ancestors? This is not an unusual question; consider that I have found a remarkably close Y-DNA match on 111 markers. What that means is that there is a 95% probability that my common ancestor with my match was alive during my 9th generation.

Most of us find new 4th and 5th cousin type matches when we do autosomal testing; interestingly, several authors feel that some of our best options will come through the spouses of our respective lines. This is a significant problem as names on the female lines are difficult to find. On many of my autosomal matches the suggested generation match is 5-8 generations.

One of my classic genealogy fails was when I went to my tree and really examined how complete and authentic my tree is for comparison purposes. I truly underestimated not only the importance of a full authentic tree but also the raw numbers of people involved. This section will have achieved its purpose if it will cause you to make a similar appraisal of your tree. My selfish reason is when I go to compare possible matches I want to have the benefit of a good tree to compare to.

I made the image just above on the left to show the calculations of the numbers of ancestors in a direct line. The number of grandparents doubles each generation starting with your two parents. Your two parents were created by four of your grandparents; and they of course were created by eight of what would be your 1G Grandparents and so on.

The numbers of ancestor grandparents you need are very impressive, the 6th generation requires 32 people, the 7th generation requires 64 people, the 8th generation requires 128 people and the 9th generation requires 256 people to fulfill their biological function of creating a child which survives long enough to be able to find a mate and reproduce.

Requiring 256 people means that 128 men needed to find 128 willing females to reproduce at least once. Do you know the names of your 128 9th generation grandfathers? How about even guessing as to the names of the 128 grandmothers who have historically forsaken their maiden names to history? Clearly each of us likely needs a more complete and authentic family tree to be able to effectively use our new DNA testing tools.

This page was last modified: Monday, 19-Nov-2018 11:29:49 MST