Thursday, January 30, 2014

The Narrative

Mitochondrial DNA (mtDNA) testing Map of human migration out of Africa, according to Mitochondrial DNA. The numbers represent thousands of years before present time. The blue line represents the area covered in ice or tundra during the last great ice age. The North Pole is at the center. Africa, the center of the start of the migration, is at the top left and South America is at the far right.
A direct maternal ancestor can be traced using mtDNA. MtDNA is passed down by the mother unchanged, to all children. A perfect match is found to another person's mtDNA test results indicates shared recent ancestry. More distant matching to a specific haplogroup or subclade may be linked to a common geographic origin. Some people cite paternal mtDNA transmission as invalidating mtDNA testing,[3] but this has not been found problematic in genealogical DNA testing, nor in scholarly population genetics studies. See the rest of this article. What gets tested
mtDNA, by current conventions, is divided into three regions. They are the coding region (00577-16023) and two Hyper Variable Regions (HVR1 [16024-16569], and HVR2 [00001-00576]).[4] All test results are compared to the mtDNA of a European in Haplogroup H2a2a. This early sample is known as the Cambridge Reference Sequence (CRS). A list of single nucleotide polymorphisms (SNPs) is returned. The relatively few "mutations" or "transitions" that are found are then reported simply as differences from the CRS, such as in the examples just below. The two most common mtDNA tests are a sequence of HVR1 and a sequence of both HVR1 and HVR2. Some mtDNA tests may only analyze a partial range in these regions. Some people are now choosing to have a full sequence performed, to maximize their genealogical help. The full sequence is still somewhat controversial because it may reveal medical information.
Y chromosome (Y-DNA) testing
A man's patrilineal ancestry, or male-line ancestry, can be traced using the DNA on his Y chromosome (Y-DNA) through Y-STR testing. This is useful because the Y chromosome passes down almost unchanged from father to son, i.e., the non-recombining and sex-determining regions of the Y chromosome do not change. A man's test results are compared to another man's results to determine the time frame in which the two individuals shared a most recent common ancestor, or MRCA, in their direct patrilineal lines. If their test results are a perfect, or nearly perfect match, they are related within genealogy's time frame.
Each person can then look at the other's father-line information, typically the names of each patrilineal ancestor and his spouse, together with the dates and places of their marriage and of both spouses' births and deaths. This information table will be referred to again within the mtDNA testing section below as the (matrilineal) "information table". The two matched persons may find a common ancestor or MRCA, as well as whatever information the other already has about their joint patrilineal ancestry prior to the MRCA—which might be a big help to one of them.[9] Or if not, both keep trying to extend their patrilineal ancestry further back in time. Each may choose to have their test results included in their surname's "Surname DNA project". And each receives the other's contact information if the other chose to allow this. They may correspond, and may work together in the future on joint research.[10] Women who wish to determine their direct paternal DNA ancestry can ask their father, brother, paternal uncle, paternal grandfather, or a male cousin who shares a common patrilineal ancestry (the same Y-DNA) to take a test for them.
SNP markers Strand 1 differs from strand 2 at a single base pair location (a C → T polymorphism). A single-nucleotide polymorphism (SNP) is a change to a single nucleotide in a DNA sequence. The relative mutation rate for an SNP is extremely low. This makes them ideal for marking the history of the human genetic tree. SNPs are named with a letter code and a number. The letter indicates the lab or research team that discovered the SNP. The number indicates the order in which it was discovered. For example, M173 is the 173rd SNP documented by the Human Population Genetics Laboratory at Stanford University, which uses the letter M. SNPs are mutations from the original and happen on the blocks of the trunk of the Y chromosome and happen much less frequently than STRs. From father to son hardly one SNP happen on all the Y chromosome trunk. A random SNP happens on average every two to three generations
United States - African ancestry
Y-DNA and mtDNA testing may be able to determine with which peoples in present-day Africa a person shares a direct line of part of his or her ancestry, but patterns of historic migration and historical events cloud the tracing of ancestral groups. Testing company African Ancestry[18] maintains an "African Lineage Database" of African lineages from 30 countries and over 160 ethnic groups. Due to joint long histories in the US, approximately 30% of African American males have a European Y-Chromosome haplogroup[19] Approximately 58% of African Americans have the equivalent of one great-grandparent (12.5 percent) of European ancestry. Only about 5% have the equivalent of one great-grandparent of Native American ancestry. By the early 19th century, substantial families of Free Persons of Color had been established in the Chesapeake Bay area who were descended from people free during the colonial period; most of those have been documented as descended from white men and African women (servant, slave or free). Over time various groups married more within mixed-race, black or white communities.
According to authorities like Salas, nearly three-quarters of the ancestors of African Americans taken in slavery came from regions of West Africa. The African-American movement to discover and identify with ancestral tribes has burgeoned since DNA testing became available. Often members of African-American churches take the test as groups.[citation needed] African Americans cannot easily trace their ancestry during the years of slavery through surname research, census and property records, and other traditional means. Genealogical DNA testing may provide a tie to regional African heritage.

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