17 June 2010

Three DNA Tests for Genealogists, part 3 Autosomal

Family Finder Testing

This test by Family Tree DNA focuses on autosomal DNA (atDNA). Autosomal DNA is found in our 22 pairs of non-sex chromosomes. The atDNA represents the accumulated DNA inheritance from your ancestors. You inherit approximately fifty percent of your genes from your mother and the remainder from your father. In turn, each of them inherited about fifty percent from each of their parents, and so on. Autosomal markers recombine or restructure themselves differently for every person at conception. In other words, these are the markers which make you look like your family, but not exactly…unless you have an identical twin. These markers make you a unique individual. They give you your mother’s high cheek bones, your father’s nose, etc.

Family Finder tests SNPs (Single Nucleotide Polymorphisms), for over 500,000 autosomal points.

Both men and woman can test their atDNA. This test finds matches for any of your cousins on your pedigree chart between the top line of the pedigree chart (Y-DNA) and the bottom line (mtDNA). However, the test has limitations as it requires a certain length of DNA in a continuous sequence to be handed down from the ancestor in order to match another tester with enough mathematical certainty to determine a level of cousin-ship. The longer the segments are that match a person, the closer the relationship. In the case of children, half-siblings, parents, uncles, aunts, first cousins, and other close relatives, there will be multiple long segments of matching DNA. With more distant cousins, those segments are much shorter. The test is most accurate in determining relationships up to the fifth cousin level. This does not mean you can only find fifth cousins or less. I have found seventh, ninth, and tenth cousins, but finding the common ancestor can be extremely difficult unless you and your match have extensive information on your lineage and not just for the direct line.

The determination of a cousin-ship is built on the amount of DNA inherited from your ancestors. As we all can inherit a different amount from each ancestor, the calculations are only mathematical probabilities which can give you a range of relationship. That is, your match may be listed as a fourth cousin, but in reality, the paper trail could show the match to be anywhere from a 3rd cousin once removed to a 6th cousin or more. The reason for this is multi-faceted, but, in general, it depends upon the amount of DNA inherited, the generational “half-steps” of being once or twice removed as well as a possibility of pedigree collapse.

Since each person inherits a different combination of their ancestors’ DNA, it can be prudent to have your parents, grandparents and cousins tested, as well. This will definitely help you narrow connection possibilities with matches. Testing the older generations will also help you find matches farther back in your lineage. As the test is able to confirm an approximate fifth cousin match, a fifth cousin for your parent or grandparent takes you farther back in time than a test for yourself. BUT, as we all inherit different amounts of autosomal DNA, you may have matches your sibling or cousin does not have. Testing more people in your extended family means a great chance for some different matches.

As all my parents and grandparents are dead, I tested myself, my son, and my paternal first cousin. The chart to the left is where my son (green) and my cousin (blue) match me. The dark sections are where no matches occurred as well as segments of the chromosomes which are not relevant to testing for genealogists.

If my cousin and I both match someone, I know the connection is on my father’s side. If only my son and I match someone, it is more likely that either the connection is on my mother’s line or that my cousin did not inherit enough of the DNA to connect with the match. Using your relatives in such a manner can narrow the hunt for your common ancestor.

However, just because you do not have a match with someone does not suggest you are not related. Gene segments recombine at random, and this means if you do not match a person you can still be related. You or your cousin did not inherited a large enough DNA segment in the same location to meet the minimum threshold to determine cousin-ship. For this reason, you will not be a genetic match to all of your genealogical cousins, should they test.

Family Finder (atDNA) summary:

· Males and females can test their atDNA in portions of the 22 pairs of chromosomes.
· The test gives matches anywhere on your pedigree chart.
· Single Nucleotide Polymorphisms (SNPs) are tested.
· The larger the segment you share with someone the closer your common ancestor.

In conclusion, it is clear that each type of test helps genealogy. Relatives can be located with any of them, but how those relatives are related to us depends upon which test is used. The Y-DNA test finds matches to the all male line which is usually the surname in most cultures. The mtDNA test finds cousins along the all female line, but is more informative about our ancient culture. The Family Finder test locates cousins everywhere else on our pedigree charts, but with confidence back to the fifth cousin although connections can be found beyond that.

With DNA testing, the genealogist can prove and disprove their paper trail, find cousins they have never met, share genealogies with the hope that the new cousins have more information, and gain new research partners. Finding genealogical cousins is the best reason to DNA test family members!

The next article will provide ideas on how best to locate the common ancestor for those you match.

copyright 17 Jun 2010, E. Aulicino
Thank you, R.

No comments: