FAMILY TREE DNA TUTORIAL

DNA is the carrier of our genetic information, and is passed down from generation to generation. All of the cells in our bodies, except red blood cells, contain a copy of our DNA.

At conception, a person receives DNA from both the father and mother. We each have 23 pairs of chromosomes. Of each pair, one was received from the father and one was received from the mother. These 23 pairs of chromosomes are known as nuclear DNA because they reside in the nucleus of every cell (except red blood cells).

The 23rd chromosome is known as the sex chromosome. As with the other chromosomes, one is inherited from the father, and one from the mother. The 23rd chromosome from the mother is always an X. From the father, a person either inherits an X chromosome or a Y chromosome. The chromosome inherited from the father determines their gender. An X from the father would result in an XX combination, which is a female, and a Y from the father would result in an XY combination, which is a male.

      

Whenever a particular base is present on one side, its complementary base is found on the other side. In the example above, see how the bases always occur in complementary pairs. Guanine (green) always pairs with cytosine (red) and thymine (yellow) always pairs with adenine (blue). So we can write the DNA sequence by listing the bases along either one of the two sides. In the example shown, one side reads:

T G T T C G T C etc.

For Genetic Genealogy, which is the application of DNA testing to genealogy research, two types of DNA can provide information useful in conjunction with genealogy research. These two types are the Y chromosome and mtDNA. The areas that we test are found in the so-called “Junk DNA” of the Y chromosome and mtDNA because it is found between the genes.

Y DNA
The Y chromosome is transmitted from father to son. Testing the Y chromosome provides information about the direct male line, meaning the father to his father and so on. The locations tested on the Y chromosome are called markers. Occasionally a mutation occurs at one of the markers in the Y chromosome. Mutations are simply small changes in the DNA sequence. They are natural occurrences and take place at random intervals. Overall, they are estimated to occur once every 500 generations per marker. Mutations can sometimes be valuable in identifying branches of a family tree.

Each marker has a name assigned to it by the scientific community, such as DYS#391, DYS#439 or GATA H4. The scientists classify these markers as Short Tandem Repeats (STR) because at each of these marker locations a short DNA code repeats itself. The result for a marker is the number of times the code repeats at that location and is called the allele value.

The result received for a Y-DNA test is a string of allele values called a “haplotype.” Here is an example of a 25-marker haplotype:

mtDNA
mtDNA is passed from mother to child. Since only females pass on their mtDNA, testing the mtDNA tells about the mother, to her mother, and so on along the direct maternal line. Both males and females receive mtDNA from their mothers, so both men and women can test their mtDNA.

While mutations occur in mtDNA, the rate of mutation is relatively slow. Over thousands of years these mutations build up so that one female line will have a sequence distinguishable from another. As people spread throughout the world, mutations occasionally occurred in different populations over time. This allows us to test the mtDNA to identify the world origin of a person’s lineage.

mtDNA is tested and the result is compared to a reference sequence called the Cambridge Reference Sequence (CRS). By comparing an mtDNA sequence to the CRS, we can identify the ancient lineage to which you belong, called the haplogroup. Many haplogroups are continent-specific and some of their branches are region-specific.

mtDNA Haplogroups
Haplogroups are labeled alphabetically. Today, anthropologists have identified certain haplogroups that originated in Africa, Europe, Asia, the islands of the Pacific, the Americas, and sometimes particular ethnic groups. Of course, haplogroups that are specific to one region are sometimes found in another, but this is due to more recent migration.

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