Genealogical DNA test

A genealogical DNA test looks at a person's genome at specific locations for the purposes of determining ethnicity and genealogical relationships. Results give information about ethnic groups the tested may be descended from and about other individuals that they may be related to. In general, they do not give information about medical conditions or diseases. Several types of genealogical DNA tests are available, with each looking at a different part of the genome and useful for different types of genealogical research.

Taking a genealogical DNA test requires the submission of a DNA sample. The most common way to collect a DNA sample, which can be done by either visiting a DNA test clinic or by ordering a home test through an independent DNA test supplier, is by a cheek-scraping (also known as a buccal swab). Other methods include spit-cups, mouthwash, and chewing gum. After collection, the sample is mailed to a testing lab.

Some laboratories, such as FamilyTreeDNA, offer to store DNA samples for ease of future testing.

There are three major types of genealogical DNA tests: Autosomal and X-DNA, Y-DNA and mtDNA. Autosomal tests look at Chromosomes 1-22 and X. The automsomes (chromosomes 1-22) are inherited from both parents and all recent ancestors.The X-chromsome follows a special inheritance pattern. Ethnicity estimates are often included with this sort of testing. Y-DNA looks at the Y-Chromosome, which is inherited father to son, and so can only be taken by males to explore their direct paternal line. mtDNA looks at the mitochondria, which is inherited from mother to child and so can be used to explore one's direct maternal line. Y-DNA and mtDNA cannot be used for ethnicity estimates, but can be used to find one's haplogroup, which is often associated with a particular ethnic group or region.

Autosomal DNA is the 22 pairs of chromosomes that do not contribute to sex. These are inherited exactly equally from both parents and roughly equally from grandparents to about 3x great-grand parents. Inheritance is more random and unequal from more distant ancestors. Generally, a genealogical DNA test might test about 700,000 SNPs (single-nucleotide polymorphisms). Like mtDNA and Y-DNA SNPs, autosomal SNPs are changes at a single point in genetic code. Autosomal DNA recombines each generation. Therefore, the number of markers shared with a specific ancestor decreases by about half each generation.

The major component of an autosomal DNA test is matching other individuals. Where two individuals share in common a number of consecutive SNPs, it can be projected that they share a segment of DNA at that part of their genomes. If the segment is longer than a threshold amount set by the testing company, then these two individuals are considered to be a match. The unit for segments of DNA is the centiMorgen(cM). For comparison, a full human genome is about 6500 cMs. Most companies will show the customers how many cMs they share, and across how many segments. From the number of cMs and segments, the relationship between the two individuals can be estimated, however due to the random nature of DNA inheritance, relationship estimates, especially for distant relatives, are only approximate. Some more distant cousins will not match at all

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