**Page under construction - changes/edits will be made on a daily basis***
The Autosomal DNA contains most of the DNA that makes us who we are. Because of the way it is transferred down the generations, we usually cannot identify which specific ancestor gave us specific genes (unless we compare our results with people we know we are related to, and see which stretches of DNA match).
Below is a diagram showing how much of our autosomal DNA we inherit from our Ancestors. As you can see, our Autosomal DNA is (theoretically) representative of our whole ancestry.
Up until a couple of years ago, - the only autosomal tests that were available for comparing between people were those using CODIS markers (ie. the DNA markers used for Crime scene DNA testing). These use only a dozen or so markers, so were of limited usefulness. These markers were never intended to be used to infer biogeographical ancestry, - but there are some companies that claim to do so and they can give very misleading results. These CODIS markers are also the same markers that are commonly used for paternity testing. They can also be used for testing other relationship (eg. if two people are siblings), but often results from these markers can be inconclusive. For interest I had my family tested with these markers and the results can be seen here.
The latest new offering in the genetic genealogy world are products like 23andMe's "relative finder"and FTDNA's "Family finder". These test more than half a million DNA SNP markers so are considerably more robust, and therefore far more scientifically valid.
Grandparent/Child inheritance diagrams
These diagrams are constructed using the genetic data of a real family. They were constructed from the 23andMe "family inheritance" function. The genome of each "child" was compared to 3 of their 4 grandparents, and the comparison diagrams were spliced together to create the diagrams below.
As you will see from the diagrams above, each child inherits 50% of their DNA from each parent (ie. the genetic contribution from both paternal grandparents adds up to 50%, and the genetic contribution from both maternal grandparents adds up to 50%), but the contribution from each individual grandparent varies from the predicted 25%... Why is this so???
On average we inherit 25% from each grandparent, but in reality it can vary anywhere from 0%-50%.
So,.. - how many DNA "coin tosses" are there??... We have 23 pairs of chromosomes, but there are there are billions of SNP's. So,.. is it 23 coin tosses? or is it billions of coin tosses?
You can read more about crossover (and Meiosis) here
Finding cousins with Autosomal DNA
At present there are two Autosomal DNA products available that are specifically designed to help with genealogy. In October 2009, 23andMe lauched the beta for "Relative finder", and in February 2010 FTDNA began its launch of "Family Finder". I will add more information about FTDNA's family finder as more information is known...
Theoretical %'s shared with different types of cousin
As we saw just before,.. there is variation in the %'s of DNA from each grandparent that are passed on by parents to children. This variation would tend to compound through the generations, so thus there would in reality be a wide range of variation. On average sixth cousins would share .01% of their DNA, but in reality it could easily vary anywhere between 0% - 1%.
Autosomal DNA can be used to estimate how related two people are - but you can see that because of the randomness that happens when each persons DNA is halved to make the egg or sperm, there is a wide degree of variation, and this large degree of variation is compounded each generation. So thus, if the amount of matching DNA that two people have is consistent with their being 5th cousins, they may be in reality more related, or less related.
Therefore if two people have a match in a tool such as 23andMes relative finder or FTDNA's Family finder that suggests that they are 5th cousins, but the common link can't be found, - then we can't really blame the tool for "making a mistake", - in a way,... if anyone is to "blame" it is our ancestors, for passing down each of their parents DNA in such a random way. Likewise, if we compare our DNA with cousins that we have already identified with conventional genealogy, we may find that the estimated relationship (from the DNA) may vary a bit from the known relationship - it may overestimate, or it may underestimate. Again, we can't "blame" tools such as relative finder for not estimating the true relationship "correctly"- it's our ancestors that are really to "blame".
Accordingly,.. FTDNA has chosen to report only to 5th cousin level - as they know that as we go back each generation, the margin of error increases, and beyond the 5th cousin level the margin of error is too great to give a reliable result.
A comparison of my 23andMe results, DecodeMe results (from upload of 23andMe data), and DNA tribes can be seen here [link to be added soon].
If you are interested primarily in Genealogy - I recommend FTDNA's "Family finder". FTDNA is primarily a Genealogy company, so any matches you might have are more likely to also be genealogists. Many of 23andMe's customers are not genealogists, so the response rate from "relative finder" matches is consequently relatively low. FTDNA's chromosome browser is also the best for comparing matching segments - it combines the best features of both of its main competitors. Like Decodeme, you can select between several different thresholds for matching segment lengths. Like 23andMe, FTDNA's "Family finder" matches you with other customers in their database that have matching stetches of DNA (and thus may have inherited the stretch from a common ancestor).
If you are interested primarily in comparing close relatives - I recommend either 23andMe or FTDNA's "Family finder". If you want to compare siblings, then 23andMe would be the preferred choice (since FTDNA doesn't distinguish between half identical and fully identical chromosomal regions, and full siblings will have some fully identical segments). If you are not interested in sibling comparisons and not interested in family inheritance of health traits, then FTDNA's "family finder" would be better value for money.
If you are primarily interested in "Racial" makeup, - I recommend 23andMe. The results from 23andMe are more scientifically robust than the results from "Ancestry by DNA", and are less confusing than the results from Decodeme. People with just European ancestry generally are found to be 100% "European".
If you are primarily interested in Health traits - I recommend either DecodeMe or 23andMe. DecodeMe's coverage of some conditions is more robust, but overall 23andMe is better value for money. If you are only interested in health traits (and not interested in Ancestry information at all), then Navigenics is another option that you might like to consider.
If you are primarily interested in "Global similarity" - I recommend DecodeMe. 23andMe's advanced global similarity is excellent,.. but DecodeMe's is beyond excellent. Get Decode me if you can afford it,.. otherwise 23andMe is better value for money.
If you are primarily interested in Entertainment, - I recommend DNA tribes. CODIS markers are the markers used in forensics (eg. CSI, paternity). The markers were selected for their ability to distinguish between individuals, not for their ability to predict biogeographical ancestry. Their application to predict ancestry is based solely on false logic, and is not scientifically valid.