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Interpretation of Results Disclaimer: Ancestry percentages obtained with AncestryByDNA™ reports of genetic affiliations, not necessarily recent genealogical histories they way you are accustomed to thinking of them. They are anthropology-driven AND genealogy-driven - a 10% East Asian result for a European population, or person, could have a very simple, genealogical interpretation (i.e. recent ancestors were Chinese) or a more complex, anthropological explanation (i.e.from an ethnic group with a historical connection to East Asians). Please refer to the “Interpreting the Results” section to understand how genetic affiliations arise in human populations and how to responsibly interpret genome base population affiliation test results such as those provided by AncestryByDNA™. Use of DNAPrint® BGA Admixture Estimates by Genealogists
There are three main sources for obtaining minority admixture in one's results: Recent exogamous admixture events Ancient affiliations with ethnic groups that are characterized by systematic admixture. The primary affiliation is with an isolated group that is genetically intermediate to the four our test is calibrated for. Each is described below in more detail: 1. The results of an exogamous event in a recent genealogical time from (since 1492 AD, the beginning of the European colonial period). For example, as shown below, a Chinese great grandparent in an otherwise homogeneous European family tree would produce a grandchild of European/East Asian admixture.
In this figure, we show the individuals that are 100% East Asian (Chinese) with red and we are interested in the admixture results for the male (square) at the bottom of the pedigree indicated with the short arrow.A person with a single 100% East Asian (i.e. Chinese) great grandparent and seven 100% European great grandparents would be expected to have 12.5% (one eighth) East Asian admixture.By the law of genetic assortment, the expected level is actually a range around 12.5%, with values several percent above and below possible.The grandparent the arrow is pointing to is about a 50%/50% East Asian/European mix and her daughter, the subjects mother, is expected to be a 25%/75% East Asian/European mix.In rare circumstances, our test can reliably detect an exogamous further back than 6-7 generations and in most cases it is useful for detecting such an event that occurred in the last 4-5 generations (i.e. 200-250 years back from present time would be the most distant recent admixture the test could detect, depending on the average generation length being used and the age of the person in the family being tested). 2.Admixture could result form ancient sources (in an anthropological time frame), which have been preserved in modern times by endogamous, relatively geographically isolated, close-knit community structure (i.e. ethnicity).For example, we know that modern day demography is shaped by not only the pre-historical migration process that led to the settlement of the globe, but by admixture between these populations throughout the globe.The map below shows these migrations as measured from Y-chromosome sequences, which occurred over many tens of thousands of years.
Admixture between the groups, after they had developed as distinct groups, has occurred many times throughout ancient and even more recent history. Generally, the focus has been on the difference between these groups, that which has resulted from the separation and spread of human groups to settle the globe. Little effort has been focused on the admixture events for the simple reason that they are more difficult to study scientifically.However, there is, from our test but also the work of other population geneticists, budding data for some of these admixture events.For example, both we and Rosenberg et al., 2003 and have found that there is significant East Asian admixture in Russians and (from our data) some Eastern Europeans.The extent to which the Mongolian and Hun invasions may have contributed to this admixture over a long time period remain a tantalizing mystery.There is even more pronounced East Asian admixture apparent for Native Americans (Rosenberg et al., 2003 and our own unpublished data), and a person with a fair number of Native Americans or Russians in their family tree could very well exhibit as much East Asian admixture as an individual with one 100% Chinese great grandmother and 7 other 100% European great grandparents!This is shown in the figure below:
On the left is a time scale showing the time the most significant migrations occurred.On the right is a very, very large family tree.The tree is for a single individual, who resides at the bottom apex of the triangle and it is so large because it goes back 60,000 years when there are tens of thousands of ancestors for this person.The time scale for the figure on the left applies to the large family tree on the right as well.The tree is the same as that shown in 1) above, only much, much larger and without the lines connecting the ancestors (each spot represents an ancestor, but there are so many of them it is not practical to show all of the lines connecting the tiny spots).The pink ancestors are “Russian”, which for the purposes here we will assume are an ethnicity that arose about 18,000 years ago.The red spots are East Asian and we assume here that the average Russian harbors 10% East Asian admixture.The gray spots are the precursors to Russians (whoever they might have been – lets say Eastern Europeans).In this example we can see that most of this persons Russian ethnicity comes from the left side of their family tree – which we can assume to be that part of the tree representative of his fathers side of the family.The point of this illustration is that if the average Russian harbors 10% East Asian admixture, and half of the person’s family tree is predominantly Russian, the person would be expected to harbor 5% East Asian admixture.East Asian admixture would be significant for this person even though neither their grandmother, grandfather or any other relative within the past 18,000 years was homogeneous East Asian.The way to visualize this on the family tree is to count all of the red spots and divide them by the total number of spots of any color in the tree – which would come to about 5%.In other words, relatively homogeneous East Asians represent about 5% of the total number of ancestors for this person. Of course, the family tree for some people involves numerous groups that are characterized by small degrees of this type of archaic admixture. Why are family trees like this one so polarized with certain ethnicities and why is it uncommon to see a tree with an equal distribution of each of the four BGA groups (West African, Native American, European and East Asian)?Until recently, and even now to a certain extent, people have tended to have children with others like them.Mainly this has been due to geographical isolation and proximity, but also in part due to social pressures and even sexual selection.What this means is that most family trees are not a mish-mash of random affiliations but are highly polarized as is the one in our example. The Pennsylvania Germans (aka Pennsylvania Dutch) are another possible example of transmission mechanism 2), where in this ethnic community, it seems there may have existed significant East Asian content prior to 1700 in its Germanic antecedents. These antecedents were mostly from communities that populated the mountain valleys in the middle and upper Rhine River basin of southern Germany. When the antecedents of the PA Germans settled in Pennsylvania they predominantly settled in the fertile valleys of the inland counties of Pennsylvania. Since these Pennsylvania communities remained relatively isolated, and the PA Germans married within their own kind, the level of East Asian admixture has remained significant to this day in some PA Germans tested.Dilution of this existing level would require repeated external admixture with other Indo-European ethnicities such as French or Sardinian, for which East Asian admixture is not detectable.In this case, the particular German antecedents that founded some of the PA German (aka PA Dutch) community may have been of substantial average East Asian admixture, which may have been due to sampling from within a more heterogeneous German population (i.e. a founder effect). This hypothesis of East Asian content in some PA Germans from an anthropological time frame source is currently being investigated by one of our customers, Mr. Charles Kerchner, via his Pennsylvania German, aka Pennsylvania Dutch, BioGeographical Ancestry DNA project. See his project http://www.kerchner.com/pa-gerdna.htm website for more information on that project. 3.mechanism 3 is more complex and we believe it applies only for certain individuals such as South Asian Indians or American Indians – that their primary affiliation is with an isolated group that is genetically intermediate to the four our test is calibrated for.For example, lets say A is East Asians, B is the Maya, Pima, Columbians and C are the Karitiana and Surul.It is known that group B show considerable affiliation with group A and C (East Asian affiliation, our data and Rosenberg et al., 2003) and that group C shares little affiliation with group A.It is also known that group A gave rise to B and C. It is unlikely that group A gave rise to B who gave rise to C, rather, currently accepted anthropology states that B and C came from A at different times.In other words, the Maya/Columbians/Pima may be from later waves of migrants across the Bering Strait and because they were derived from East Asians more recently, they still retain more East Asian sequences than descendants of earlier waves such as the Karitiana and Surul. Most genealogists are interested in the type of admixture found in source mechanism 1), where the admixture gives information about geopolitical affiliation of recent ancestors, rather than anthropological information on distant ones. This is because
Therefore, genealogists tend to seek information such as that which might be produced from mechanism 1 - that is, due to recent admixture. For example, if a person is trying to prove or disprove an American Indian rumor or legend in their family, a 10% Native American admixture result would be very useful if it could be assured that the mechanism for this admixture was mechanism 1 not 2.The problem is that it is not possible for us to distinguish between mechanism 1 and 2 (or even 3 for certain people) simply knowing the results of the BGA test, so our 4-group admixture test is not the end to a genealogists journey, but rather it is another important piece for the genealogists puzzle that should be considered in light of other evidence. Genetically, there is no difference in a 10% Native American admixture result from mechanism 1 or 2, but to a genealogist the distinction is important.Fortunately for a genealogist, the hard work they have gone through can come to bear in a sort of “meta” analysis.Indeed, for some genealogists, depending on their family tree, there might exist evidence that strongly suggests whether mechanism 1 or 2, or a combination of the two is the most likely explanation for an admixture result. For example, for a person whose family has paper evidence of a Native American great grandparent, it would seem logical to assume that a 10% Native American admixture result probably arose due to recent admixture as suggested by the paper genealogy.Alternatively, for a person of genealogically confirmed and homogeneous European ancestry, a 10% East Asian admixture result probably arose due to mechanism 2. Negative results carry different meaning than positive ones for genealogists. For example, if there is circumstantial, but low quality data suggesting a pure blood West African grandfather, a test result of 100% European would seem to discount that rumor (taking into account the genetic law of independent assortment, of course which would make such a result possible but highly unlikely if the genealogical data was in fact correct). However, if a person’s family is suspected to have had a Chinese great grandfather, one cannot prove it from a 20% East Asian admixture result, since it is not possible to distinguish mechanism 1) from mechanism 2). Rather, the two data combine to paint a portrait of likelihood – that it is most likely that the persons paper genealogy is correct and that the suspected Chinese great grandfather is most likely the source of the 20% East Asian admixture – in other words that it arose from mechanism 1) rather than 2) (though it could also have arisen from a combination of 1) and 2)). In summary, for a genealogist, it is important to bring other information to bear in order to reconstruct the most likely source for an admixture result. In fact, the BGA admixture data serves as an independent clue for someone attempting to reconstruct their family history, and when it is used with genealogical knowledge, the two combine to form evidence that is more powerful than either on their own. This being said, it is important for a genealogist to consider the quality of their paper data when attempting to perform such a meta-analysis. If the paper data is of low quality, the source for a good quality BGA admixture result may be obscured or misconstrued. Fortunately, many genealogists pour endless hours retracing their roots, and for these, it would seem that data quality is as good as is possible to obtain. As with all genealogical research, the preponderance and quality of the evidence should be used to form one’s conclusions. The DNAPrint® test provides another tool and piece of evidence. But one must always keep in mind the two most common possible sources of minority affiliation: style="mso-spacerun: yes"> 1) a recent in a genealogical time frame exogamous event, or 2) much older anthropological sources being carried within the population group for many centuries by endogamous customs of that group such as described above. Acknowledgments. The above information is being provided at the suggestion and with input from Charles Kerchner, a genealogist, in our efforts to help genealogists to better understand how the DNAPrint® test can be used as a tool to aide genealogists in their research.
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