Autosomal DNA (atDNA) is recombined randomly between atDNA of both parents. The unit of measurement of shared segments is the CentiMorgan (cM): “It is defined as the distance between chromosome positions (also termed, loci or markers) for which the expected average number of intervening chromosomal crossovers in a single generation is 0.01”
Segments below 5cM might be identical by state (IBS), not identical by descent (IBD). Identical twins have a 6722.2cM match, Parents a 3400cM match; Siblings average 2640cM match; Grandparents, Aunts, Uncles, ½ siblings- 1700cM; Great Grand Parents, 1st Cousins, Great Uncles- 850cM; 1st Cousins once removed- 425cM; 2nd Cousins, 1st Cousins 2x Removed- 212.5cM; 3rd Cousins, 2nd Cousins 2x Removed- 53.13cM; 5th Cousins- 3.32cM; 8th Cousins- .05cM
atDNA Products include FTDNA ‘s “FamilyFinder” and 23andMe, AncestryDNA, etc. Most of these companies sell Cousin matching (reliable to 5th Cousins and very occasionally out to 7th and 8th), and usually offer a very generic Biogeographic Analysis or “Ethnic Composition”
Where this becomes interesting is where you can download your raw data, and use at a site like GEDmatch.com against population genetics databases, and ancient DNA to make chromosome paintings like these:
The only problem is, the percentages vary depending on what was in the source/comparison database, so you can never take the percentage as universal- it’s just what percent was shared with that group in the comparison database. This has also led to an under-reporting of Native American heritage, as for various legal and sociological reasons many of that culture are opposed to testing and are therefore not found in many comparison data sets. The previous 4 graphics were all made with the same autosomal data, just against different databases.
One further complication is that due to recombination, all of your true genealogical ancestors are not going to be found in your atDNA. Since you can picture atDNA as two decks of cards that are shuffled between your parents, then half discarded, leaving only one 52 card deck going forward- repeat that with 2 more decks over many generations and shuffles the two hybrid decks and discard half gain, repeat with another 2nd gen hybrid deck, etc.. and there may be no cards from one of the original decks left in the many-many times descendant deck. Random chance determines some of this, so mixed population people like many Americans will have this problem with confirming distant ancestors admixtures.
Part 7 will conclude with mtDNA