Genetic Genealogy: Combining the use of DNA testing and Genealogy is a relatively young method of family research. …In the late 1990's, there were several highly publicized cases, i.e.: the "Cheddar Man", Thomas Jefferson and Sally Hemmings, and the last Czar of Russia's family, to name a few, in which DNA was utilized to prove or disprove relationships to people that have long since been deceased. The media coverage of these, and other cases, helped to bring DNA testing for genealogical applications to the commercial market in the year 2000. …The International Society of Genetic Genealogy (ISOGG) was founded in 2005 by DNA project administrators who shared a common vision: the promotion and education of genetic genealogy. (2018 Sources: https://isogg.org/wiki/Wiki_Welcome_Page ; and https://isogg.org/wiki/Portal:DNA_testing)
Cells, Chromosomes and DNA: DNA is found in every cell in your body except red blood cells. In the center of each cell is a membrane called a nucleus. A nucleus contains chromosomes, and chromosomes are made up of long strands of DNA which contain all the body's genes. (Genes are the functional units of DNA.) Humans have a total of 46 chromosomes, which are grouped into pairs. Each of the 23 pair consists of one chromosome from our mother and one from our father. In females the 23rd chromosome pair consists of two X-chromosomes. Males, however, have an X-chromosome and a Y-chromosome. Therefore, it is the Y-chromosome that determines male gender. (2008 Sources: Sorenson Molecular Genealogy Foundation and Brough Family Organization)
Y Chromosome DNA (Y-DNA) tests: The Y-Chromosome is one of the 23rd pair of human chromosomes. 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. 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 very close, they are related within a genealogically useful time frame. A surname project is where many individuals whose Y-chromosomes match collaborate to find their common ancestry. (2018 Source: https://en.wikipedia.org/wiki/Genealogical_DNA_test)
Mitochondrial DNA (mtDNA) tests: The mitochondrion is a component of a human cell, and contains its own DNA. Mitochondrial DNA usually has 16,569 base pairs (the number can vary slightly depending on addition or deletion mutations) and is much smaller than the human genome DNA which has 3.2 billion base pairs. Mitochondrial DNA is transmitted from mother to child, thus a direct maternal ancestor can be traced using mtDNA. The transmission occurs with relatively rare mutations compared to the genome 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. (2018 Source: https://en.wikipedia.org/wiki/Genealogical_DNA_test)
Autosomal DNA (atDNA) tests: 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 exactly equally from both parents and roughly equally from grandparents to about 3x great-grand parents. Therefore, the number of markers (one of two or more known variants in the genome at a particular location – known as Single-nucleotide polymorphisms or SNPs) inherited from a specific ancestor decreases by about half each generation; that is, 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, etc. Inheritance is more random and unequal from more distant ancestors. Generally, a genealogical DNA test might test about 700,000 SNPs (specific points in the genome). (2018 Source: https://en.wikipedia.org/wiki/Genealogical_DNA_test)
DNA Tests Will Not Match All Of Your Tested Cousins: "Currently it’s never been found that a 2nd cousin will not share DNA , so if you are not sharing DNA with a known second cousin (who has also tested) then you have a mystery to solve, something is not right. But more distant than 2nd cousin and it’s possible to no longer have any DNA from the distant ancestor. This is generally around 10% of 3rd cousins who will not match you, up to 50% of 4ths and 70% of 5ths. It can vary of course, but these are high level numbers, so don’t be surprised if a known 4th cousin does not share DNA with you, it does not mean your paper trial is wrong and more research is needed to confirm the branch." (2017 Source: Donna Rutherford, DNA - What, When, How, Why - FAWs for Beginners)
Non-Relatedness: DNA tests sometimes suggest that people who once thought they were related are not so related. Such an unexpected finding of "non-relatedness" may reflect an adoption, an altered or assumed surname, an illegitimate birth, or maternal infidelity somewhere in the ancestral line. In addition, one must keep in mind that the science of genetic genealogy is relatively young, and there is still much that scientists are learning about human ancestry and its migrations over time, unusual DNA anomalies, and the extrapolation and statistical probabilities of specific ancestral DNA relationships through time. (2017 Source: Brough Family Organization)
What If Your DNA Test Does Not Support Your Genealogical Assumptions? Always remember that "Family is family, whether it is by blood, adoption or inheritance." If DNA testing does not support your genealogical assumptions, do not distance yourself from those who have supported and loved you during your life. Regardless of how you received or acquired your surname--whether it was by blood, adoption or inheritance--stay close to those who know and love you, and invest in strengthening family ties that connect you to those you call and know as "family". (2017 Source: Brough Family Organization)
Additional Online Information about DNA Tests
2014: Understanding Patterns of Inheritance: Where Did My DNA Come From (And Why It Matters), by Anna Swayne, via Ancestry.com. This article is about Autosomal DNA tests and their results.
2017: DNA - What, When, How, Why - FAWs for Beginners, by Donna Rutherford. This article provides information information and understanding about genetic DNA testing.
2017: Still Not Soup, by Judy G. Russell. This article presents the concerns, limitations and problems surrounding DNA ethnicity estimates.
2018: The following DNA companies were displayed and highlighted at RootsTech 2018: Ancestry DNA, FamilyTree DNA, Living DNA, MyHeritage DNA, and 23andME DNA.
2018: Is DNA testing telling us more than we want to know? The untold story of Ancestry.com, by Erica Evans, Deseret News, May 30, 2018.
2018: Genome Hackers Show No One's DNA Is Anonymous Anymore, by Megan Molteni, Science, October 11, 2018.
2021: DNA Testing, by the International Society of Genetic Genealogy (ISGG). This article and others by ISGG contains extensive information about DNA tests.