Saturday, December 21, 2013
DNA and Its Role in Genealogy
In order to provide an overview of the various DNA tests available for genealogical research, I have compiled the following information in hopes that it will be helpful to those readers who are contemplating such testing.
For the purpose of genealogical research, several DNA tests are appropriate. These tests are the Y-chromosome (Y-DNA), the mitochondrial (mtDNA), and the autosomal (atDNA). The purpose of this document is to provide a basic understanding of the types of tests and what they reveal.
A copy of one’s DNA is present in all of his bodily cells except the red blood cells and is passed down to each of his succeeding generations. Each individual has 23 pairs of chromosomes (one of each pair is from the mother and one from the father) with the 23rd pair being the sex chromosome.
The 23rd chromosome from the mother is always an X since she does not have a Y chromosome. An X or a Y may be inherited by the fetus from the father. If the X is inherited from the father, the fetus will be a female with two X chromosomes (XX). If the Y is inherited from the father, the fetus will be a male with one of each chromosome (XY).
The two X chromosomes of the mother randomly swap information and genes. The X chromosome that a fetus receives from its mother is a mixture of the X chromosomes from her parents. The X that a fetus may inherit from its father is that of the father’s mother and is a mixture of his mother’s two X chromosomes, one that she received from her father and one that she received from her mother. Therefore, the mixture in that X chromosome that the fetus receives from its father is from its maternal grandparents.
The Y Chromosome DNA Test (Y-DNA) is a test only for males. It analyzes the male Y chromosome which is transmitted ONLY from father to son. Therefore, the Y-DNA test provides information about the direct male line from son to father, to his father, to his father, etc. The results are reported in markers. Several Y-DNA tests assess different amounts of markers. Family Tree DNA offers the following Y-DNA tests: the Y-DNA37 which assesses 37 markers and provides matches that are likely to be related with the past 8 generations; the Y-DNA67 which assesses 67 markers and provides matches that are likely to be related within the past 6 generations; and the Y-DNA111 which assesses 111 markers and provides matches that are likely to be related within the past 4 generations. Also, with the Y-DNA test, a haplogroup is predicted. Male halpogroups are labeled with the letters A through T. The haplogroup provides the participant with information regarding the major population group from which he descends.
Participants in the Y-DNA testing program may join a FTDNA surname project in which those with the same surname may share information.
The Mitochondrial DNA Test (mtDNA) may be taken by males and females. The mitochondrial is a small circle of DNA found inside the cells and has only 37 genes. It does not recombine and is, therefore, passed on from the mother to the child without change. The mitochondrial (mtDNA) is passed from the mother to each of her children, male or female. Only females can pass the mtDNA to their children. The mtDNA test reveals information about one’s direct maternal line from one’s mother, from her mother, from her mother, etc. The mtDNA test identifies the world origin of one’s lineage and the haplogroup to which he or she belongs. These haplogroups are continent-specific and/or region-specific. The female haplogroups are identified with the letters of the alphabet depending on the system used by the testing company. Family Tree DNA uses the Cambridge Reference Sequence, which is the accepted mtDNA standard, using the following letters of the alphabet with their corresponding regions: J, K – Southern Europe; H, T, U, V, X – Northern Europe; J, N – Middle East; L, L1, L2, and L3 – Africa; A, B, C, D, F, G – Asia; and A, B, C, D, and sometimes X – Native American. Those interested in learning more about the female haplogroups will find The Seven Daughters of Eve by Bryan Sykes, a genetics professor at Oxford University, very informative.
The mtDNA test is available in the mtDNA Plus, a mid-level assessment, and the mtDNA Full Sequence, the highest level of assessment. The mtDNA Plus reports results in two hypervariable regions, HVR1 and HVR2. The matches included with these two regions are related to the past 28 generations. The mtDNA Full Sequence reports results in all three regions, HVR1, HVR2, and the Coding Region. Matches are related within the past 16 generations.
The Autosomal Test (atDNA) is appropriate for both men and women and provides male and female matches that are related within the last 5 generations. However, matches beyond 5 generations are frequently found, but the confidence level set by FTDNA for the Family Finder test is 5 generations. Family Finder is capable of identifying matches in any branch of one’s family and is not limited solely to the paternal or maternal lines. In addition, it provides percentages of one’s ancestral lineage such as Native American, Middle-Eastern, Jewish, African, and Western and Eastern European. In Family Tree DNA, the autosomal test is known as Family Finder.
The analyses of one’s 22 chromosomes (not the 23rd one) are reported in terms of centimorgans (cMs), longest blocks, and single nucleotide polymorphism (SNPs) which is the most common type of genetic variation among people. One may compare this information with other matches using graphics and spreadsheets which may be accessed through the FTDNA’s Family Finder site. In addition, relationship-ranges and estimated-relationships are predicted for each of one’s matches. One must understand that the DNA he inherits from his parents (50% from the father and 50% from the mother) does not represent 25% from each of the 4 sets of grandparents or 12.5% from each of the 8 individual grandparents but is a random mixture. However, the fact that long blocks often “stick together” for several generations makes matching possible.
Close relatives will share larger fragments of DNA from a common ancestor and smaller segments from more distant ancestors. Even these small segments may hold valuable clues for the genealogist.
One cannot compare the results of the various tests. Each test analyzes a different segment of the DNA and reports the results using different terminology. The haplogroups for men with the Y-DNA test and those for women with the mtDNA tests are different and cannot be compared.
Family Tree DNA stores one’s DNA for 25 years so that one may upgrade to a higher level of testing without submitting another sample. The number of one’s matches is dependent on the size of the company’s data base. Also, if one’s lineage or surname is rare, he may not have any matches at first, but over time as the data base grows, participants with whom he matches will become available.
Family Tree DNA and 23andMe were the first two companies to offer autosomal DNA testing. Family Finder through Family Tree DNA is specifically recommended for genealogist and adoptees. 23andMe offers an autosomal DNA test called Relative Finder, which is similar to the Family Finder developed by Family Tree DNA. Recently, 23andMe came under government scrutiny due to its offering health-related information based on the participant’s genetics without the benefits of professional medical attention. Therefore, 23andMe no longer provides health-related information to its participants.
Ancestry.com started offering genetic testing in 2011.
DNA testing can be used to confirm one’s “paper trail,” to determine which family trees with the same or variant surnames are related, to determine individuals who are or are not related, and to provide clues for further research.
“23andMe Halts Genetic Health Reports,”
“About Genetic Genealogy,” http://www.thegeneticgenealogist.com/about-genetic-genealogy/
Hill, Richard. “Autosomal DNA Testing for Genealogy,”
Powell, Kimberly. “Autosomal DNA Testing for Genealogy,” About.com Guide.
Smolenyak, Megan Smolenyak and Ann Turner. Trace Your Roots with DNA. Holtzbrinck