Genealogical DNA test

A genealogical DNA test is a DNA-based genetic test used in genetic genealogy that looks at specific locations of a person's genome in order to find or verify ancestral genealogical relationships, or to estimate the ethnic mixture of an individual. Since different testing companies use different ethnic reference groups and different matching algorithms, ethnicity estimates for an individual vary between tests, sometimes dramatically.
Three principal types of genealogical DNA tests are available, with each looking at a different part of the genome and being useful for different types of genealogical research: autosomal, mitochondrial, and Y-chromosome.
Autosomal tests may result in a large number of DNA matches to both males and females who have also tested with the same company. Each match will typically show an estimated degree of relatedness, i.e., a close family match, 1st-2nd cousins, 3rd-4th cousins, etc. The furthest degree of relationship is usually the "6th-cousin or further" level. However, due to the random nature of which, and how much, DNA is inherited by each tested person from their common ancestors, precise relationship conclusions can only be made for close relations. Traditional genealogical research, and the sharing of family trees, is typically required for interpretation of the results. Autosomal tests are also used in estimating ethnic mix.
MtDNA and Y-DNA tests are much more objective. However, they give considerably fewer DNA matches, if any, since they are limited to relationships along a strict female line and a strict male line respectively. MtDNA and Y-DNA tests are utilized to identify archeological cultures and migration paths of a person's ancestors along a strict mother's line or a strict father's line. Based on MtDNA and Y-DNA, a person's haplogroup can be identified. The mtDNA test can be taken by both males and females, because everyone inherits their mtDNA from their mother, as the mitochondrial DNA is located in the egg cell. However, a Y-DNA test can only be taken by a male, as only males have a Y-chromosome.

Procedure

A genealogical DNA test is performed on a DNA sample obtained by cheek-scraping, spit-cups, mouthwash, or chewing gum. Typically, the sample collection uses a home test kit supplied by a service provider such as 23andMe, AncestryDNA, Family Tree DNA, or MyHeritage. After following the kit instructions on how to collect the sample, it is returned to the supplier for analysis. The sample is then processed using a technology known as DNA microarray to obtain the genetic information.

Types of tests

There are three major types of genealogical DNA tests: Autosomal, Y-DNA, and mtDNA.

Autosomal DNA tests look at chromosome pairs 1–22 and the X part of the 23rd chromosome. The autosomes are inherited from both parents and all recent ancestors. The X-chromosome follows a special inheritance pattern, because females inherit an X-chromosome from each of their parents, while males inherit an X-chromosome from their mother and a Y-chromosome from their father. Ethnicity estimates are often included with this sort of testing.
Y-DNA looks at the Y-chromosome, which is passed down from father to son. Thus, the Y-DNA test can only be taken by males to explore their direct paternal line.
mtDNA looks at the mitochondria, which is passed down from mother to child. Thus, the mtDNA test can be taken by both males and females, and it explores one's direct maternal line.

Y-DNA and mtDNA do not produce a direct ethnicity estimate, but allows to find one's haplogroup. Haplogroups can only provide information on one line of ancestors among many. While they are unevenly distributed across ethnicities, their historical distribution is only speculation. Direct-to-consumer DNA test companies have often labeled haplogroups by continent or ethnicity, but these labels may be speculative or misleading.

Autosomal DNA (atDNA) testing

Testing

Autosomal DNA is contained in the 22 pairs of chromosomes not involved in determining a person's sex. Autosomal DNA recombines in each generation, and new offspring receive one set of chromosomes from each parent. These are inherited exactly equally from both parents and roughly equally from grandparents to about 3x great-grandparents. Therefore, the number of markers inherited from a specific ancestor decreases by about half with each successive generation; that is, an individual receives half of their markers from each parent, about a quarter of those markers from each grandparent; about an eighth of those markers from each great-grandparent, etc. Inheritance is more random and unequal from more distant ancestors. Generally, a genealogical DNA test might test about 700,000 SNPs.

Reporting process

The preparation of a report on the DNA in the sample proceeds in multiple stages:

identification of the DNA base pair at specific SNP locations
comparison with previously stored results
interpretation of matches
Base pair identification

All major service providers use equipment with microarray chips supplied by Illumina. The chip determines which SNP locations are tested. Different versions of the chip are used by different service providers. In addition, updated versions of the Illumina chip may test different sets of SNP locations. The list of SNP locations and base pairs at that location is usually available to the customer as "raw data". The raw data can be uploaded to some other genealogical service providers to produce an additional interpretation and matches. For additional genealogical analysis the data can also be uploaded to GEDmatch. Raw data can also be uploaded to services that provide health risk and trait reports using SNP genotypes. These reports may be free or inexpensive, in contrast to reports provided by DTC testing companies, who charge about double the cost of their genealogy-only services. The implications of individual SNP results can be ascertained from raw data results by referring to SNPedia.com.

Identification of matches

The major component of an autosomal DNA test is matching other individuals. Where the individual being tested has a number of consecutive SNPs in common with a previously tested individual in the company's database, it can be inferred 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. Unlike the identification of base pairs, the data bases against which the new sample is tested, and the algorithms used to determine a match, are proprietary and specific to each company.
The unit for segments of DNA is the centimorgan. For comparison, a full human genome is about 6500 cM. The shorter the length of a match, the greater are the chances that a match is spurious. An important statistic for subsequent interpretation is the length of the shared DNA.

Interpretation of autosomal matches

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. Although information about specific SNPs can be used for some purposes, the key information is the percentage of DNA shared by two individuals. This can indicate the closeness of the relationship. However, it does not show the roles of the two individuals, e.g., 50% shared suggests a parent/child relationship, but it does not identify which individual is the parent.
Various advanced techniques and analyses can be done on this data. This includes features such as In-common/Shared Matches, Chromosome Browsers, and Triangulation. This analysis is often required if DNA evidence is being used to prove or disprove a specific relationship.

X-chromosome DNA testing

The X-chromosome SNP results are often included in autosomal DNA tests. Both males and females receive an X-chromosome from their mother, but only females receive a second X-chromosome from their father. The X-chromosome has a special path of inheritance patterns and can be useful in significantly narrowing down possible ancestor lines compared to autosomal DNA. For example, an X-chromosome match with a male can only have come from his maternal side. Like autosomal DNA, X-chromosome DNA undergoes random recombination at each generation. There are specialized inheritance charts which describe the possible patterns of X-chromosome DNA inheritance for males and females.

STRs

Some genealogical companies offer autosomal STRs. These are similar to Y-DNA STRs. The number of STRs offered is limited, and results have been used for personal identification, paternity cases, and inter-population studies.
Law enforcement agencies in the US and Europe use autosomal STR data to identify criminals.

Mitochondrial DNA (mtDNA) testing

The mitochondrion is a component of a human cell, and contains its own DNA. Mitochondrial DNA usually has 16,569 base pairs and is much smaller than the human genome DNA which has 3.2 billion base pairs. Mitochondrial DNA is transmitted from mother to child, as it is contained in the egg cell. Thus, a direct maternal ancestor can be traced using mtDNA. The transmission occurs with relatively rare mutations compared to autosomal DNA. A perfect match found to another person's mtDNA test results indicates shared ancestry of possibly between 1 and 50 generations ago. More distant matching to a specific haplogroup or subclade may be linked to a common geographic origin.

Test

The mtDNA, by current conventions, is divided into three regions. They are the coding region and two Hyper Variable Regions.
The two most common mtDNA tests are a sequence of HVR1 and HVR2 and a full sequence of the mitochondria. Generally, testing only the HVRs has limited genealogical use so it is increasingly popular and accessible to have a full sequence. The full mtDNA sequence is only offered by Family Tree DNA among the major testing companies and is somewhat controversial because the coding region DNA may reveal medical information about the test-taker