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About Autosomal DNA

Autosomal DNA represent the fastest-growing DNA tests for genealogy  As of July 2013, about 19% of Taylor Family Genes members had purchased auDNA tests from FTDNA. This page presents a brief overview.

The term is often abbreviated "atDNA"; here, we'll use "auDNA".

Advantages and Disadvantages

Advantages

  1. Tracks all family lines, not just direct paternal and maternal.
  2. Can identify cousins, out to the the fourth cousin or, sometimes, fifth cousin level.
    • Some users have reported genealogically significant matches beyond 5th cousins, but these are not to be counted on.

Disadvantages

  1. Limited as to number of generations
  2. Databases for matching are presently smaller than the sizes needed for "blind testing" and of either Y-DNA or mtDNA. (The databases are growing rapidly.)
  3. Complicated; requires advanced genetic knowledge.
  4. May require testing multiple family members to interpret matches with unknown cousins.

The Concept

Autosomal refers to the 22 chromosomes that are not gender-determining, though some tests also include the X chromosome. We have two copies of each chromosome, one from Mom and another from Dad

DNA is composed of pairs of four bases -- A (adenine), C (cytosine), G (guanine) and T (thymine) -- AT, CG, etc. These are known as "base pairs" and are arranged in sequences along the chromosomes.

Base pairs are sometimes (incorrectly) referred to as SNPs, but SNP means that more than one form is observed and this isn't necessarily the case with base pairs; some exist in only one form.

By reading the sequences, an identifying signature is determined for particular segments. The identifying signatures are then searched across an entire database of comparable reads and identical segments are reported as matches. In general, longer identical segments indicate closer relationships and shorter identical sequences indicate more distant or no relationships.

See also Roberta Estes' blog and her paper.

auDNA and Taylor Family Genes

Autosomal DNA has little to no association with any surname. If your family is typical, each of your ancestors in almost every generation had a different surname. For most participants, Taylor will be only one of 32 or more surnames represented in matches. Each genetic family will be unique to the individual participant.

Presently, Taylor Family Genes does not take an active role in autosomal DNA; it is primarily an individual endeavor. Project administration is not involved with your auDNA results or matches.

Nor, are project administrators especially skilled in interpreting the complexities of AuDNA. If you have a question, however, we will attempt to get an answer for you.

Other Resources

What is auDNA?

Humans have 23 pairs of (46) chromosomes contained within the nucleus of each cell. One pair of chromosomes (XX or XY) determines gender. The other 22 pairs are called "autosomes" and their dioxyribonucleic acid is autosomal DNA.


Note that the chromosomes are paired;
there are two Chr4s, two Chr10s, two Chr13s, etc.

Autosomal DNA governs most of our physical characteristics and bodily functions. It determines the color of our eyes and hair and many other things about us.

AuDNA Inheritance

At conception, we receive one half-pair of each chromosome from our mothers and the other half from our fathers, making a full pair of each.

auDNA Dilution

So, we get roughly one-half our autosomal DNA from each parent. And each of them got one-half from each of their parents. As the table below shows, this rapidly dilutes the DNA received from each individual ancestor.

auDNA from each ancestor
Ancestor Generations # of
Ancestors
~% DNA
Received
Other
Relationships
Yourself 0 1 100% N/A
Parent 1 2 50% Sibling
Grandparent 2 4 25% Aunt/ Uncle, Half Sibling,
Double 1st Cousin
Great-grandparent 3 8 12.50% 1st Cousin,
1/2 aunt/uncle,
1/2 nephew/niece
2nd-great-grandparent 4 16 6.25% 1st cousin
once removed
3rd-great-grandparent 5 32 3.12% 2nd cousin
once removed,
1st cousin
twice removed
4th-great-grandparent 6 64 1.56%  
5th-great-grandparent 7 128 0.78% 3rd cousin
6th-great-grandparent 8 256 0.39%  
7th-great-grandparent 9 512 0.20% 4th cousin
8th-great-grandparent 10 1,024 0.10%  
9th-great-grandparent 11 2,048 0.05% 5th cousin

By the time we get to generation 11, the DNA from each ancestor is highly diluted with the DNA from all the others. In general, it's estimated that beyond the 7th-generation ancestor (5th-great-grandparent) the unique DNA we carry from each ancestor is almost undetectable.

auDNA Recombination

But, dilution is not the end of the story. As the autosomes  (and XX) find each other and pair up, they swap genetic information with their partners in a process known as recombination. Mom's copy of Chr1 will interact with Dad's copy and vice versa, jumbling the inherited information.

Some information on one chromosome is over-written by information from its mate. This means we may get a little more or less than half our auDNA from one parent or the other.

The two halves of each pair look a little more like each other than they did before. That results in an inheritance pattern like this:

Recombination is why full siblings other than identical twins are a little different from each other.

Spaghetti Analogy

Dr. Bruce Walsh used this analogy to explain auDNA inheritance:

  1. Take two pots of cooked spaghetti, one pot of red (tomato flavor) and one green (spinach flavor). they can be small pots, you only need 23 strands of each color.
  2. Pour the pots out on a table together and chop them into random lengths with a cleaver.
  3. Mix well.
  4. Put half in one pot, the other half in another pot. One pot will have a little more red, the other a little more green.
  5. For the next generation, cook two more pots of different colors. Repeat steps two to four.

Net Result

Dilution and recombination make auDNA matching complicated and limit how many generations it can look back.

Testing

The three major players in auDNA testing (which facilitate matching) their advantages and disadvantages are

Also see ISOGG's comparison, which covers -- in addition -- the NGS Geno 2.0 project and BritainsDNA Chromo2 test. 

Matching

As with other types of DNA used in genetic genealogy, the process with auDNA is

  1. Get a test and have results entered in a searchable database;
  2. Find matches -- those whose DNA is sufficiently similar as to indicate a recent common ancestor;
  3. Communicate with the matching persons to identify common ancestor(s).

Note: At this time, the databases are relatively small for finding matches at random. The largest, for example, represents less than 1/1,000th the population of the United States and much less than 1/1,000,000th the world population. Users may need to recruit their matches.

What is a "match"?

About 99.5% of our individual DNA is the same as almost everyone else in the world. Therefore, auDNA matching focuses on the subtle differences in the other one-half percent; it looks for people with whom you share more than 99.5%.

A match consists of persons who have sufficiently long strands of identical (or, half-identical) DNA. In general, the longer the identical strands, the closer the relationship.

However, identical strands can be classified as IBD or IBS:

In general, IBS matches tend to be on shorter sequences than IBD matches.

Phasing

Phasing is the method for determining whether autosomal matches are on your mother's side (phase) or your father's. Technically, "Phasing is the task or process of determining the parental source of a SNP's alleles." It usually requires at least two sets of results from known relatives -- one on the paternal side, one on the maternal.

Segment length

Segments are typically measured in centiMorgans, abbreviated cM.

Cousins

A match points to a contemporary (hopefully living) cousin who descends from an ancestor you share with him or her. The number of such cousins grows at an exponential rate as the number of generations of separation increases. At the same time, the amount of shared DNA increases, also at an exponential rate, decreasing the percentage that can be detected through matching. The net effect is that the number of detectable cousins grows almost linearly.

Most of the cousins found via auDNA matches will be distant, 5th degree or more. That's because their number grows faster with generations of separation than the detection rate shrinks.

Origins

One intriguing feature of autosomal DNA testing is that it can estimate your ancestral origins by place. The diagrams below depict the ancestral origins of one member of Taylor Family Genes.

 

The image on the left shows the members to be "99% European"; the missing 1% probably represents the margin of error in the estimate. Note that there are three centers of focus: British Isles, Scandinavia and Southern Europe (Balkans to Iberian Peninsula) The image on the shows relative percentages:

The biggest surprise for the member was the relative size of the Scandinavian component, that it outweighed the British Isles component.

Limitations

Autosomal DNA can not find all your cousins, for reasons state more fully at https://medium.com/@dl1dl1/face-it-dna-cannot-find-all-your-relatives-f68089b8e1e9#.ene7dwkrx. .

You have a lot of cousins, "..190 third cousins, 940 fourth cousins and 4,700 fifth cousins, 23,000 six cousins and so on. For every degree of cousinship increase, we expect five times more cousins."

But, in order to match, you must share at least one DNA segment with them. The chances are less than you might think and decrease with each step.

Relation Prob. share
IBD segment
2C 100%
2C-R 100%
3C 98%
3C-R 88%
4C 69%
4C-R 48%
5C 30%
5C-R 18%
6C 10%
6C-R   6%
  probability of sharing an IBD segment  
("C" means cousin; "R" means once removed.)

The chances of sharing even a single DNA segment with a 5th cousin are only 30% and with a 6th cousin only 10%. Similarly, you have only a 10% chance of inheriting a DNA segment that your 4th-great-grandparent once had.

Technical problems

The above would hold even if technology could read the DNA completely accurately. With the current state of the art, some errors are inevitable; they become more frequent -- and computationally difficult to resolve -- as the segments get smaller. (Segment-slicing is inherent in autosomal DNA inheritance.)


Revised 02 Mar 2016