Mar 17, 2017

DNA Overview

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Genealogists have traditionally used documents such as census records, vital statistic records, wills, parish records, etc. to determine the ancestry of a person. These documents are sometimes referred to as a "paper trail" of ones ancestry. These records may identify a person's ancestry for a number of generations, but eventually all family trees reach the point where the paper trail stops. This is the current situation with our Leigh line in Wales. We suspect the line goes back to England, but we have not found documents that prove that connection. We are thus turning to DNA testing to obtain information about our ancestors that will give us a general idea of where in England and when those ancestors lived. This page gives an overview of DNA testing.

In DNA testing, information from a person's DNA is obtained and is placed in a database. The information is then compared with the DNA of other donors, and if a match (either an exact match or a near match) is found, the approximate number of generations back to the closest ancestor that is common to both people is calculated. If no matches are found, the information is retained in the database for use in future comparisons with others. Thus, one goal of DNA testing is an estimation of the approximate time in generations back to the closest common ancestor of two people. Our hope is that distant relatives in England will have their DNA tested and that the comparison of our DNA with their DNA will provide a connection between the Leighs in Wales and the Leighs in England.

Note: None of the DNA information taken during the testing contains personal information about the person. The DNA that is tested comes from "junk DNA", DNA that is called "junk" because it has no useful information about the donor. The only information extracted from the person's DNA is information that is helpful  in determining when a potential common ancestor lived.

Y-DNA Testing 
When an embryo is conceived, two chromosomes are combined to determine the gender of the embryo. These chromosomes are known as X and Y chromosomes. The mother always contributes an X chromosome, but the father may contribute either an X or a Y chromosome. If the father contributes an X chromosome, the offspring will be a female. If the father contributes a Y chromosome, the offspring will be a male. The Y chromosome contributed by the father is passed from father to son, normally without change although infrequent changes or mutations may occur to the Y-chromosome.
Because the Y-chromosome is passed from father to son,  that chromosome can be used to identify father-son relationships and thus genealogical relationships. The testing of the Y chromosomes is called Y-DNA testing. For example, Ralph Leigh is currently our earliest Leigh male ancestor in Wales. His Y-chromosome was passed to his son, Richard, and from Richard to his two sons, and so on. Allen Leigh, the webmaster of this site, is a direct descendant of Ralph Leigh and thus has Ralph's Y-chromosome. However, Allen's Y-chromosome may not be exactly the same as Ralph's Y-chromosome, because mutations might have occurred as the chromosome was passed from father to son. Thus, in using Y-DNA to locate descendants of an ancestor, we are looking for matches between the Y-chromosome in a person who is known to be a descendant and the Y-chromosome in persons who may or may not be descendants, with the realization that because of mutations the matches may not be exact.
Markers are short segments of DNA, and Y-DNA testing involves comparisons of these markers. In the example given above of Ralph Leigh and Allen Leigh, Ralph has certain values for his markers. Likewise, Allen has particular values for his markers. If the two sets of markers are exactly the same, there is a high probability that Ralph and Allen are related. However, if differences (caused by mutations) exist between the two sets of markers, the probability decreases of Ralph and Allen being related. The results obtained from Y-DNA testing include a set of markers and the values of those markers. This data is useful only when comparisons are made between two or more people. Y-DNA testing can be done for various amounts of markers, such as 12, 25, or 37 markers. Having more markers involved in the testing means a more accurate estimation of the relationship can be made. In our example of Ralph Leigh, since we don't have Y-DNA from Ralph, we use Y-DNA from two or more living persons who are believed to be descended from Ralph.
Genetic Distance 
The "closeness" of a relationship between two people that was determined via Y-DNA testing is expressed as the Genetic Distance of those persons. Genetic Distance is the number of differences between the markers of the people. For example, if two people have 5 differences in their markers, they have a Genetic Distance of 5. This could be one marker that differed by 5. It could also be 5 markers that differed by 1, or any other combination of markers & differences, such that the number of differences is equal to 5. If two people have a perfect match, their genetic distance is 0.
Most Recent Common Ancestor 
If two men are related, there must be at least one person in their ancestral lines that is common to both persons. The closest person who is common to the two men is known as the Most Recent Common Ancestor or MRCA of the two people, and the number of generations from a person to his or her MRCA is known as the Time to MRCA or TMRCA. In the case of Y-DNA testing, the markers of two men who are believed to be in unbroken male lines from a common ancestor are compared, and the TMRCA for the two men is calculated. It is important to understand that the calculated TMRCA is an upper limit, and the common ancestor could have lived at any time within the range specified by the TMRCA.

Mutations to the Y-DNA occur infrequently and there may be several generations between changes to the Y-DNA. Since Genetic Distance is the number of changes that have occurred, a larger number of mutations requires a larger amount of time to have expired since the two lines separated, resulting in a common ancestor who probably lived a larger number of generations before the persons being tested.

Y-DNA Haplogroups 
When a man has his Y-DNA tested, he is told the name of his Haplogroup. The Haplogroup identifies the origin of the male line and thus gives information about the major population group of the line.
mtDNA Testing
There is another type of DNA test that has been used by some genealogists. This test is known as mtDNA and involves DNA that is passed from a mother to all children. Men receive this DNA but do not pass it to their children. This test is not commonly used by genealogists because of the social custom of married women taking the surname of their husbands, thus making it difficult to have good correlation between the results of this maternal test and the paternal surname lines commonly used in genealogy. However, the use of this test is becoming popular among genealogists, because it gives another view of the relationships in genealogical research.

By Allen Leigh

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