A genealogical DNA test is a DNA-based test which 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.
These test provide individual ancestral route information, and can help connect people with their relatives, sometimes as distantly as fourth or fifth cousins.
This information can be informative to a person who does not know their genealogical ancestry.
Genetic ancestry testing is being used to help identify or exclude criminal suspects, and can be used to inform healthcare decisions.
GAT involves the comparison of large numbers of DNA variants measured in an individual with the frequencies of these variants in reference populations from across the world.
Geographic regions in which an individual variant is highest in frequency is assumed to be the most likely location of an ancestor who transmitted the variant to the person being tested.
GAT is traditionally done for mitochondrial DNA which is transmitted only by females and reflects the origin of one maternal ancestors and for Y chromosome DNA transmitted from father to son and reflecting the origin of one paternal ancestor.
An assessment of ancestry can be conducted by assaying a half million or more autosomal variants (single nucleotide variants (SNVs), formally single-nucleotide polymorphisms (SNP’s), which are inherited from both parents.
The specimen is processed using a technology known as DNA microarray to obtain the genetic information.
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 autosomes (chromosomes 1–22) are inherited from both parents and all recent ancestors.
The X-chromosome follows a special inheritance pattern.
Ethnicity estimates are often included with autosome 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.
Y-DNA and mtDNA can be used to find one’s haplogroup, which is unevenly distributed geographically.
Haplotype groups labeled by continent or ethnicity may be speculative or misleading.
Autosomal DNA is contained in the 22 pairs of chromosomes not involved in determining a person’s sex.
Autosomal DNA recombines each generation, and new offspring receive one set of chromosomes from each parent.
These are inherited autosomal DNA chromosomes are exactly equally from both parents and roughly equally from grandparents to about 3x great-grand parents.
The number of markers, variants in the genome at a particular location – known as single-nucleotide polymorphisms or SNPs are inherited from a specific ancestor decreases by about half each generation.
An individual receives half of their markers from each parent, about a quarter of their markers from each grandparent; about an eighth of their markers from each great grandparent, and so on!
Inheritance is a more random process and unequal from more distant ancestors.
Genealogical DNA tests about 700,000 single nucleotide polymorphisms (SNPs).
The unit for segments of DNA is the centimorgan (cM).
The full human genome has about 6500 cM.
The longer the length of a match, the greater are the chances that a match is not spurious.
From the number of cMs and segments, the relationship between the two individuals can be estimated, it they are only approximate.
Some more distant cousins will not match at all.
The key information is the percentage of DNA shared by 2 individuals, indicating the closeness of the relationship.
By counting the percentage of single nucleotide polymorphisms (SNP’s) originating from each geographic region, the percentage of an individual‘s ancestry derived from each region can be estimated.
X-chromosome DNA testing: Both males and females receive an X-chromosome from their mother, but only females receive a second X-chromosome from their father.
An X-chromosome match with a male can only have come from his maternal side.
X-chromosome DNA undergoes random recombination at each generation.
Father to daughter X-chromosomes are passed down unchanged.
Ancestry can be designated broadly such as western Asia, southern Europe, or as fine as by regions within individual countries.
The accuracy of ancestral information is affected by the choice of reference populations and the selection and number of singular nucleotide variants.
Some genealogical studies include autosomal STRs (short tandem repeats), used for personal identification, paternity cases and inter-population studies, criminal IDs.
The mitochondrion of a human cell, and contains its own DNA.
Mitochondrial DNA usually has 16,569 base pairs, much smaller than the human genome DNA which has 3.2 billion base pairs.
Mitochondrial DNA is transmitted from mother to child, therefore a direct maternal ancestor can be traced using mtDNA.
Mitochondrial DNA transmission occurs with relatively rare mutations compared to the genome DNA.
A perfect match found to another person’s mtDNA test indicates shared ancestry of possibly between 1 and 50 generations ago.
More distant matching may be linked to a common geographic origin.
All humans descend in the direct female line from Mitochondrial Eve, a female who lived probably around 150,000 years ago in Africa.
Only males have a Y-chromosome, because women have two X chromosomes in their 23rd pair.
A man’s patrilineal ancestry, or male-line ancestry, can be traced using the DNA on his Y chromosome (Y-DNA), because the Y-chromosome is transmitted father to son nearly unchanged.
Men’s test results are compared to
other men’s results to determine the time frame which individuals shared a most recent common ancestor (MRCA), in their direct patrilineal lines.
Women who wish to determine their direct paternal DNA ancestry can ask their father, brother, paternal uncle, paternal grandfather, or a paternal uncle’s son to take a test for them.
There are two types of DNA testing: STRs and SNPs.
Most common is STRs (short tandem repeat).
A section of DNA is examined for a pattern that repeats.
The number of times it repeats is the value of the marker.
STRs mutate fairly frequently, comparing results of two individuals are then compared to see if there is a match, to estimate of how closely related two people.
A single-nucleotide polymorphism (SNP) is a change to a single nucleotide in a DNA sequence.
Typical Y-DNA SNP tests test about 20,000 to 35,000 SNPs.
All human men descend from the paternal line are from a single man dubbed Y-chromosomal Adam.
Adam lived probably between 200,000 and 300,000 years ago.
A family tree can be drawn showing different branches are different haplogroups.
Most haplogroups can be further subdivided multiple times into sub-clades.
Some known sub-clades were founded in the last 1000 years.
Generally the world is specified into about 20–25 regions, and the approximate percentage of DNA inherited from each can be noted.
The frequency of each Autosomal DNA marker is tested and compared to many population groups.
Ethnicity estimates have become more accurate.
People who test for traditional genealogy often utilize a combination of autosomal, mitochondrial, and Y-Chromosome tests.
Matching on the mtDNA full sequence is used to confirm a common ancestor on the direct maternal line between two suspected relatives.
Autosomal tests usually report the ethnic proportions of the individual, and
they attempt to measure an individual’s mixed geographic heritage by identifying ancestry informative markers (AIM), that are associated with populations of specific geographical areas.
Approximately 30% of African American males have a European Y-Chromosome haplogroup, and approximately 58% of African Americans have at least the equivalent of one great-grandparent of European ancestry.
Only about 5% of African-American males have the equivalent of one great-grandparent of Native American ancestry.
Substantial families have been established in the Chesapeake Bay area who were descended from people during the colonial period; most of those have been documented as descended from white men and African women.
Nearly three-quarters of the ancestors of African Americans taken in slavery came from regions of West Africa.
African Americans usually cannot easily trace their ancestry during the years of slavery through surname research, census and property records, and other traditional means, so genealogical DNA testing may provide a tie to regional African heritage.
Autosomal testing, Y-DNA, and mtDNA testing can be conducted to determine the ancestry of Native Americans.
Genetic ancestral testing may establish whether a person’s direct female line belongs to one of the canonical Native American Haplogroups, A, B, C, D or X.
The vast majority of Native American individuals belong to one of the five identified mtDNA Haplogroups, providing evidence of potential Native American descent.
The Cohanim (or Kohanim) is a patrilineal priestly line of descent in Judaism.
The ancestor of the Cohanim is Aaron, brother of Moses.
Descent from Aaron is verifiable with a Y-DNA test.
Study in genealogical Y-Chromosome DNA testing found that a significant percentage of Cohens have distinctively similar DNA, rather more so than general Jewish or Middle Eastern populations.
Cohens tended to belong to Haplogroup J, with Y-STR values clustered unusually closely around a haplotype known as the Cohen Modal Haplotype (CMH).
This finding could be consistent with a shared common ancestor of the hereditary priesthood having originally been founded from members of a single closely related clan.
46.1% of Kohanim carry Y chromosomes belonging to a single paternal lineage (J-P58*) that likely originated in the Near East well before the dispersal of Jewish groups in the Diaspora.
There is av relatively high frequency of J-58* in Jewish populations (»20%) and Kohanim (»46%) and its vanishingly low frequency in our sample of non-Jewish populations that hosted Jewish diaspora communities outside of the Near East.
Genealogical DNA tests allows then determination with high accuracy whether one is related to another person.
Autosomal DNA combined with genealogical research has been used by adoptees to find their biological parents,
to find the name and family of unidentified bodies and by law enforcement agencies to apprehend criminals.
Though genealogical DNA tests are not designed mainly for medical purposes, autosomal DNA tests can be used to analyze the probability of hundreds of heritable medical conditions,
A correlation exists between a lack of Y-DNA marker DYS464 and infertility.
A correlation exists between a between mtDNA haplogroup H and protection from sepsis.