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Understanding genealogy through DNA analysis by John Thompson

September 25, 2012 Blog, Genealogy Comments

By John Thompson

The good news is that your entire family history is stored securely and unambiguously in your DNA.

The bad news?

It’s not so easy to decipher, and DNA covers only the identities of your ancestors, omitting all other details on their lives. In the near future, genealogists can expect in depth information from DNA, but to understand this, a little knowledge of genetics is required (but not much).

Of the three billion positions in human DNA, any pair of unrelated people has exactly the same DNA sequence at about 99.8% of those three billion positions. Related individuals have an even higher level of identity. Close family relations can be confirmed by comparing the DNA sequence at positions that tend to vary more often than average. The differences can be either in the sequence of DNA present or its length.

It is relatively easy to determine identity — such as in forensics, or very close relationships such as those determining paternity. However, it becomes progressively more difficult to analyze more distant relationships.

Embedded in nearly all the cells in the body each person has 23 pairs of chromosomes with one of each pair contributed by the mother and one pair by the father. Of these 23 pairs, 22 are virtually identical. One of the pairs is the sex chromosome. Each individual gets one X from their mother and either an X or a Y from their father. Thus, the sex chromosome in the individual is either XX (female) or XY (male). In addition, most cells have hundreds of copies of mitochondrial DNA that is much shorter than the chromosomal DNA and comes only from the mother.

If the DNA behaved itself and tracked nicely from one generation to the next, molecular genealogy would be easy. There are changes in the DNA, however, in every generation. (If you think transcribing census records is touchy, try getting 3,000,000,000 base pairs of DNA right every time.)

The individual chromosomes in the pairs also recombine with each other, mixing up the parents’ contributions in each generation. Thus, each of your great-grandparents supplied one-eighth of your DNA, but their contributions are scattered throughout all of your chromosomes and not so easy to track, especially since each of your great-grandparents was 99.8% identical to the others to begin with!

There are a couple of special cases that are easier to deal with. The simplest DNA to look at is the mitochondrial DNA (from only the mother) and the Y specific DNA (from only the father). More has been done with mitochondrial DNA because it is easier to work with. It is much shorter and there are many more copies of it, but it can only be used to trace maternal lineages. If you and the person of interest share the same mitochondrial DNA sequence, you must have the same maternal ancestor some generations back.

Not enough has been done to know how much identity is needed to prove common ancestry because, as mentioned earlier, DNA sequence changes slowly as you travers generations. A very detailed account of how this type of information was used to identity century-old bones is presented in The Romanovs: The Final Chapter, by Robert Massie.

Y-specific DNA can be used to trace paternal lineages. Since the mother does not have a Y chromosome, the father’s contribution remains “pure.” One good example of using Y DNA was confirmation of a relation between Thomas Jefferson and the offspring of Sally Hemings (see the scientific journal NATURE, Volume 396, p. 27).

It is important to note that in both the Romanovs and Jefferson/Hemings examples, DNA alone was not used to prove a genealogical relationship. A lot of research went into documenting historical data and providing a specific hypothesis that was then put ot the DNA test. It is likely that this will remain the predominant use of the technology for the foreseeable future.

The use of DNA to establish broader ethnic/national heritage relationshiops is more doable in the near future, but the quality of the connections is highly dependent on well documented populations that are needed as a reference.

For more information about DNA, the human genome project, and related topcis, visit these sites:

The Guide to Understanding Genetics by the National Institute of Health

Basic and Applied Genetic Research from the Oak Ridge National Laboratory

To contact John Thompson (email).

Originally printed in the DFA…. Edited by Carol Doane.

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