Background on DNA Testing

Background on Y-DNA Testing


Those who had high school biology will remember that humans have 23 pairs of chromosomes (for a total of 46 chromosomes). The chromosome pair designated 23 is special and determines the sex of the individual. When forming the chromosome pair 23, if the individual receives an X chromosome from the mother and an X chromosome from the father, the individual is female (XX). If the individual receives an X chromosome from the mother and a Y chromosome from the father, the individual is male (XY). The Y chromosome is passed from the father to the son virtually unchanged.


The Y in Y-DNA testing refers to the Y chromosome passed from a father to a son. The reason Y-DNA testing is especially useful for genealogy is that the Y chromosome is passed intact from father to son down through the generations in the same way surnames are passed in most cultures. Here is a Descendant Chart that shows how male A passes on his Y-chromosome.



Image of Descendant Chart

In the above Descendant Chart for ancestor A, he passes on his Y-chromosome to the males designated in dark blue squares. Note that it’s common to represent the males by squares and the females by circles. For the males B and C, male A is the most recent common ancestor (MRCA). If males B and C both took Y-DNA tests, their results would show a close match. Therefore, a family history researcher could conclude that the two are closely related through a MRCA.



Image of Pedigree Chart for Henry J. Jackson, Jr.

The above diagram is a Pedigree Chart for Henry J. Jackson, Jr. that shows his parents and grandparents. In a Pedigree Chart, the top most person in each column is a father that passed on his Y-chromosome to his son all the way to down to the further most person on the left (assuming the person is male). These individuals have been colored in blue in the diagram and are referred to as the “Y-Chromosome Line.” Notice that Y-DNA testing is only useful for direct male lineal descent (father to son to grandson, etc.), i.e., the blue colored boxes. It says nothing about the many descendants through the female lines.


For example, going back five generations, you have in your Pedigree Chart 62 ancestors (2 parents, 4 grandparents, 8 great grandparents, 16 great-great grandparents and 32 great-great-great grandparents) assuming no cross marriages such as with two first cousins marrying. Assuming you are male, Y-DNA testing only supplies information about the five direct male ancestors (your father, his father, his father, etc.) along the Y-Chromosome Line (the blue boxes in the above Pedigree Chart). A Y-DNA test says nothing about the other 57 ancestors!


However, even with this severe limitation, the Y-DNA test can be useful for genealogy. For example, a family history researcher could use it to determine if two branches of a surname share a common ancestor. The researcher just has to identify a male from each surname branch that descends through the male lineage back to the common ancestor. Most researchers would identify two males from each branch in case one individual might have a “hidden” adoption or an infidelity (called a Non-Paternal Event or NPE). Y-DNA tests deal with strictly male blood lines.


The “67” in the Y-DNA67 test refers to the number of markers used in the test. In genetic genealogy (the name for using genetics for genealogy), DNA labs use completely different markers from the ones used by the police at crime scenes as seen on CSI and the ones used by the medical profession for susceptibility to diseases. If one is considering a Y-DNA test, he need not worry about the police knocking on his door or about revealing health issues. Y-DNA tests specific markers on a male’s Y-chromosome known as Short Tandom Repeats, or STR markers. The number of markers tested by most DNA testing companies can range from a minimum of 12 to as many as 111, with 67 being commonly considered a useful amount. Having additional markers tested will generally refine the predictive period in which two individuals are related, helpful for affirming or disproving a genealogical connection on the direct paternal line. Testing for more markers increases the cost. Testing for Y-DNA67 currently costs about $250 per individual; however, Family Tree DNA recently had a sale where they charged $200.


Background on mtDNA Testing


In the above Pedigree Chart, a pink arrow labeled mtDNA Line travels down from Henry J. Jackson, Jr. by the lowest female in each column (colored pink). Every individual (male and female) inherits his or her mother’s Mitochondria, i.e., organelles outside the nucleus of the cell that converts food into energy for the cell to live. Males don’t pass on their Mitochondria; only females. These Mitochodria have their own DNA (called mtDNA) different from the DNA in the 46 chromosomes inside the cell’s nucleus. You can have your mtDNA tested at Family Tree DNA and other companies if you wish. It’s a different test from a Y-DNA test with its own price (about $160). However, mtDNA testing is of limited use for family history. Scientists, especially anthropologists, use mtDNA to trace the migration patterns of people over thousands of years.


Background on Autosomal DNA Testing — Family Tree DNA’s “Family Finder”


FTDNA’s Family Finder provides analysis of a person’s ethnic percentages and connects them with relatives descended from any of their ancestral lines from approximately the most recent five to six generations. Because Family Finder uses autosomal DNA (inherited from both of your parents, your four grandparents, your eight great-grandparents, etc.), it offers the greatest opportunity to discover your recent genetic cousins.


Other Resources

For more details on DNA testing visit Blair Genealogy’s great web site: DNA 101