Native American Ancestry Finder
From ISOGG Wiki
This feature scans a person's Ancestry Painting for distinctive signatures that indicate a Native American ancestor up to five generations in the past. It also takes into account the maternal and, if available, paternal lines, looking for Native American ancestry at any depth along those two branches of the family tree. Generally, it will only discover Native American ancestry within the last five generations of the participant. It uses two or three lines of evidence in assessing a person's likelihood of Native American ancestry (depending on whether a person is male or female). Each of those lines appears below, along with its significance with regard to Native American ancestry:
1. Ancestry Painting - The assessment is made on the basis of extensive simulations, which make a number of simplifying assumptions.
2. mtDNA Haplogroup - Determines whether the mitochondrial DNA haplogroup is consistent with Native American ancestry along the maternal (mother's mother's mother's...) line.
3. Y-DNA Haplogroup - Determines whether the Y-DNA haplogroup is consistent with Native American ancestry along the paternal (father's father's father's...) line. Because paternal ancestry is determined by the genetics of the Y chromosome, there is no way to trace a woman's male lineage using her own DNA. But if a woman's father, brother or paternal uncle is also a user of 23andMe, that person's paternal ancestry will be the same as hers. Order a kit for her dad or brother.
People who identify themselves as Native American exhibit fairly consistent Ancestry Painting proportions of about 75% Asian and 25% European, plus or minus 10%. The reason: Native Americans descend from a small number of people who crossed the Bering land bridge from Siberia more than 14,000 years ago. In Ancestry Painting, Siberians and people from many other central and northern Asian locales tend to have a roughly three-to-one proportion of Asian to European DNA simply because they lie geographically and thus genetically intermediate between the Asian reference population, which consists of Japanese and Han Chinese individuals, and the European reference population, which consists of Americans of northern European descent.
Simulations are run to determine what would happen to that three-to-one Asian/European proportion over the generations if a Native American and a partner of all-European descent had a child, who then reproduced with another all-European partner, and so on. In such a case, the amount of Asian DNA in each successive generation's Ancestry Painting would necessarily diminish, until at some point it disappeared altogether.
To get an empirical idea of how this process works, 23andMe simulated SNP genotypes for 1,000 people with one Native American and one European parent, then 1,000 people with one Native American parent and three European grandparents, and so on down the line. Then we ran all these simulated individuals through Ancestry Painting, and looked at the range of Asian and European DNA percentages for each kind of relationship. (23andMe also looked at the African percentages for these simulations, but those were nearly always zero, or a trace at most.)
23andMe determined that it takes at least five generations after the appearance of a Native American for the percentage of Asian (orange) DNA to reach zero. They claim with confidence to be able to determine that a person with an all-European Ancestry Painting (actually, 99.74% European or greater) did not have any genetically Native American ancestors in the past five generations.