Understanding Autosomal DNA (atDNA) Testing

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There are many sites that explain DNA testing. This page describes how family members can use DNA testing to support the specific goals of research projects in which Kevin Ireland is involved. People with other goals should refer to more generalized resources.

Human Chromosome Pairs.

Understanding DNA Inheritance

Every person has 23 pairs of chromosomes. One member of each pair comes from each parent (via an egg and a sperm cell). The first 22 pairs are called autosomes, and are examined using atDNA tests from vendors like AncestryDNA, 23andMe, Family Finder, MyHeritage, and others. I recommend AncestryDNA because they have a much larger database of people who’ve tested - and who may match. Using a single vendor also lets us compare lists of matches across multiple family members.

The final pair of chromosomes are called the sex chromosomes. Females inherit one 'X' chromosome from each parent to make up their 23rd pair. Males inherit one 'Y' chromosome from their father and one 'X' chromosome from their mother. Information about the 'Y' chromosome (which only men have) can be inspected using one of several types of Y-DNA tests. I recommend Y-DNA testing from FamilyTreeDNA.

Autosomal DNA (atDNA) Key Points

Here are a few things to remember about atDNA inheritance:

• Our atDNA is a subset of our parents'.
• atDNA can't skip a generation.
• Everyone inherits a random 50% of each parent's atDNA.
• This means we DON'T inherit the other half of each parents' atDNA.
• Because all of our father's atDNA came from his parents, this means we get half of our atDNA from our paternal grandparents. However, the mix we inherit from each individual paternal grandparent often varies from 20 to 30% (even though it sums to 50%). Same goes for our maternal grandparents.
• Each of our siblings inherit a randomly different half of each parent's atDNA (except for identical twins, who have identical DNA).

These inheritance patterns have significant effects on atDNA tests - because atDNA is effectively watered down by an average of 50% each generation. After 4 to 8 generations, it becomes less and less likely to match your distant cousins - even when you share two common ancestors (such as your 6th great-grandparents).

Autosome 3 Example - Mother

Autosomal Recombination - Chromosome Pairs 1-22

When an egg (or sperm) cell is formed in the parent's body, each of their pairs of autosomes has to be reduced to a single autosome in a process called recombination. (Once the sperm fertilizes an egg cell, the autosomes are paired - with one autosome from each parent). This happens identically for each autosome, but let's consider how it might work for autosome 3 (as it does for every other one).

Autosome 3 Example - Mother's Egg Cell

The mother inherits one entire autosome 3 from her mother (labeled GM for grandmother) and one from her father (labeled GF for grandfather). When her body creates an egg, it combines location-specific segments from each autosome pair to create a new, unique, single autosome for that egg. The number and lengths of the segments are relatively random - and differ on each autosome (and in each egg).

In this hypothetical example, the newly-created autosome contains three segments. The first 32% of the egg's autosome 3 is a segment that the mother inherited from her mother (GM; the zygote's grandmother). The next 58% was inherited from her father (GF), and the final 10% was a segment from her mother (GM). Note that this means that the future child's first autosome 3 will contain 42% of its genetic material from its maternal grandmother and 58% from its maternal grandfather.

Autosome 3 Example - Father

Autosome 3 Example - Father's Sperm Cell

Similarly, the father's autosome 3 pair contains one autosome inherited from his mother and one autosome inherited from his father. When his body generates a sperm cell, it similarly a creates a single autosome 3 that (usually) contains genetic segments from each of his parents. Every sperm cell contains a different mix of genetic material in each autosome.

This theoretical example shows that the newly-created autosome 3 contains five segments of genetic material; three segments from his father (GF) and two segments from his mother (GM).

Larger autosomes, such as autosome 3 usually contain segments from each parent (or from the zygote's perspective, grandparent). However, it's common for smaller autosomes (such as 16 to 22) to get all of their genetic material from only one parent/grandparent.

Autosome 3 Example - Resulting Fertilized Egg (aka Zygote)

Autosome 3 Example - Zygote

While the typical women releases an average of about 450 unique mature egg cells during ovulation in her lifetime, men release 40M to 1.2B unique sperm cells in each ejaculation. The segments on each autosome in each egg and each sperm cell are different from the other. Autosomes average 50% of genetic material from each grandparent, but most of them vary from 40% to 60% or beyond.

After the egg cell is fertilized by the sperm, the Autosome 3 from the egg is paired with the Autosome 3 from the sperm to create the autosome pair - so the resulting zygote always contains 50% of its genetic material in each of the 22 autosomes from each parent. However, the contribution from the zygote's grandparents can vary fairly significantly. The "average" contribution from each grandparent is 50%, but the variance in any individual grandchild can be quite wide. This is one of the reasons that Ethnicity estimates for full siblings can differ even though they have identical ancestral lines.

Gender Determination - Chromosome 23 pair: X and Y

When a man forms a sperm cell, it contains a copy of either his entire 'X' chromosome or his entire 'Y' chromosome. If the chromosome 23 in the sperm is an 'X', the fertilized egg will become a female. If the chromosome 23 is a 'Y', the fertilized egg will become a male. In either case, it will be paired with a recombined 'X' chromosome in the egg that comes from the mother.

Sex Chromosomes

The DNA inheritance process is identical to the example given above for all 22 autosomes. The 23rd chromosome pair, often called the "sex chromosomes" are passed on differently.

Females have two 'X' chromosomes - one inherited from their mother and one inherited from their father. When their bodies create eggs, they generate a new unique X chromosome in the same manner as new autosomes described above.

Males have one 'X' chromosome inherited from their mother and one 'Y' chromosome inherited in its entirety from their father. When men generate sperm cells, each sperm cell contains either their entire 'X' chromosome (inherited from the sperm's grandmother) or their entire 'Y' chromosome (inherited sperm cell's grandfather). If the sperm cell that fertilizes an egg contains the 'Y' chromosome, the resulting zygote becomes a male. If it contains the 'X' chromosome, the resulting zygote will become a female.

X-DNA can be considered in genealogy research, but that's an advanced topic that isn't covered here.

Y-DNA is crucial for genealogy research, because the Y-chromosome is passed down basically unchanged (with only occasional minor mutations) from father to sons for thousands of years.

Autosomal DNA testing - AncestryDNA

Autosomal DNA (atDNA) tests are available to men and women, and help users estimate how closely they're related to each other. If both users have a good understanding of their family tree, they can often determine their most likely common ancestor(s).

Users "match" if they share a significant identical segment on the same location on one or more autosomes. Closer relatives match on longer segments, and often share matches on multiple segments. The likelihood of a match is based on shared DNA amounts measured in centimorgans - and number of shared segments.

For example, all three of my father's sisters have tested on AncestryDNA. I match to them with:

• GP - 2,032cM on 46 segments
• SI - 1,773cM on 49 segments
• SM - 1,901cM on 45 segments

The common ancestors I share with them are my paternal grandparents (and my aunts' parents). Even though my biological relationship to each of my dad's sisters is identically "nephew," I inherited different lengths of DNA (on different autosomes) from our common ancestors than they did. In most cases, shared DNA amounts support or refute a genealogical relationship rather than "proving" it.

As we look at more distant relationships, the randomness of shared DNA amounts becomes more evident. For example, my father shares 240cM over 10 segments with my third cousin AF. I share just 9.2cM over 2 segments with her. Researchers estimate that about 10% of all 3rd cousins don't match - and that more than 25% of 4th cousins don't match.

This is actually one of the strongest arguments for testing multiple cousins and siblings. AF may have inherited multiple segments from our common ancestors that I did not. It's likely that her siblings or mine would match on some of these.

Autosomal DNA Inheritance

The regular price for an AncestryDNA test in the US is $99 plus $9.95 shipping (includes pre-paid return shipping). A number of times a year, the test goes on sale for $39 to $59. US residents can buy the test on Amazon and usually get free shipping. Note that AncestryDNA sales are country-specific.

The Amazon purchase link to buy the test at the current US price with free shipping (whether regular $99 price or lower sale price) is usually https://www.amazon.com/AncestryDNA-Genetic-Ethnicity-Test/dp/B00TRLVKW0/. If you buy on Amazon, verify first that the price is as low as the current price at AncestryDNA.

Y-DNA testing - FamilyTreeDNA

Although multiple vendors sell Y-DNA tests, most people use FamilyTreeDNA. They have the largest database of Y-DNA tested men in the world, making it easier to identify matches. FamilyTreeDNA makes their tests available worldwide (except where the sale is prohibited by local law). Visit https://www.familytreedna.com to purchase tests or upgrades.

To learn more about Y-DNA testing, read Introduction to Y-DNA Testing.