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Introduction to DNA


Everyone has 23 pairs of chromosomes in the nucleus of each cells. These contain most of your DNA (some is in your mitochondria). One pair is special because they are linked to your gender. All females have one pair where each chromosome looks under a microscope like the letter X. All males have one that looks like X and the other that looks like Y.  With each new generation, the DNA in the chromosomes gets mixed around between that of the mother and father. Since only males have Y-chromosomes, the DNA on the Y-chromosome doesn’t become mixed in each generation. Each male inherits his “y” only from his father. That is why the Y-DNA test is useful for identifying the parental line.

When the Y-DNA is copied from the father to the son, errors happen in some parts of the long DNA chain. The DNA consists of a series of letters A, T, G, and C representing slightly different groups of molecules sticking off like hairs from the strand of DNA. These form the ‘steps’ of the double helix that you see in images of DNA. When these are copied, the copies are not perfect. A number of locations on the family-tree1chain are particularly prone to copy errors because they contain repeated sequences like ATATAT. The copying of DNA for the son may make a mistake in the number of the repeats, so it may contain more or less of the repeated sequence called Short Tandem Repeats (STR). The error prone regions of the DNA always occurs at the same place so that locations can be numbered just like street signs. When you examine the chart for the project results, you see numbers like DYS393 and DYS390 along the top. These are the street signs showing where on the Y-chromosome the SRT is located. The numbers such as 13 or 14 in the column mean that at that location the STR was 13 or 14 units of repeating DNA. There is no set order in which STR street signs are presented; we use the Family Tree DNA ( order convention for the first 111 numbers (markers) used in our project because Family Tree DNA is the primary lab for STR testing.

There is nothing special about the exact numbers, but the larger the fraction of them that match between two people, the higher the chance the two people are related. If you have two people who match all 111 STR’s, then they are certainly close relatives through their fathers. Where there are less than perfect matches, it is possible to calculate a probability that the two are related and how close the common ancestor might be.

A second type of mutation that occurs in copying changes a single letter of the DNA and can happen anywhere on any chromosome. These mutations are represent unique event polymorphism (UEP) or more commonly called Single Nucleotide Polymorphisms (SNP). A letter may be copied incorrectly or even deleted. An example of a SNP mutation would be if DNA that read  ‘ATCGTA’ in the father was copied as ‘ATCCTA’ in the son. The original G has become C. These mutations can have health consequences, but usually they are benign. Each of us carries millions of SNP’s in our DNA.

DNA-SNP Markers

Examples of SNP Markers

Scientists have compiled SNP mutations in the Y-DNA from people all over the world and have used that to develop a rough estimate of when the major mutations occurred. This is called the Haplotree, and each mutation causes a new branch of the tree to form. The base of the Haplotree is in Africa where modern humans first appeared. That trunk is labeled Haplogroup A, and people whose ancestors never migrated out of Africa are in this group. The first branch that forms is labeled ‘B’ and represents a specific SNP mutation. A further mutation leads to branch ‘C’ and so forth. The Henderson Clan is predominately composed of people from the ‘I’, ‘J’, and ‘R’ branches with a few in others as well.

There is one additional factor in the UEP’s that is relevant to the Henderson Project. It is possible for a mutation to change back to its original form. This is thought to be rare. One location, labeled M-17 contained the letter A in the earliest humans and also in chimpanzees. Early in the evolution of humans, a unique mutation occurred that changed this to G. This mutation created the ‘B’ branch of the Haplotree, and all subsequent branches have letter G at M17. However, roughly 15,000 years ago, a back mutation occurred that changed the G back to A. This is a rare event and most statistical analysis of SNP’s assume that back mutations do not occur. The ancestral leader of Clan Donald, Somerled and his descendants have this back mutation. Our family group R-A1 also appears to have this back mutation that separates them from the rest of the Henderson R group families.


Example of Halpo Groups

It is possible to estimate approximately when the SNP mutations that gave rise to the different branches, Haplogroups, occurred. Just looking at the letters, you know the ‘I’ branch separated before the ‘J’ and much before the ‘R’. The ‘R’ branch may have arisen as recently as 4000 years in the past. In some cases, it is possible to use genealogical information to determine exactly when a mutation occurred. In Clan Donald, genealogists have been able to show that the R-CLD12 SNP mutation originated in the year 1352 with the birth of Ranald mac Donald, 1st of Clanranald and Glengarry. The Stewart Clan has determined that a mutation labeled R-S781 occurred between the birth of the first son of Alexander in 1243 and the second son in 1245. Unfortunately, the Henderson clan does not yet have enough data to define our SNP history in this detail. Furthermore, the details of the Haplotree and identification of SNP’s is an ongoing research effort with a constantly expanding list of branches.

SNP’s are also sometimes related to health. The DNA testing service 23&Me is building a database that attempts to connect specific SNP’s with susceptibility to various health issues. This is still very much a work in progress and depends on collecting both DNA and health data from a large number of users.

The other type of DNA you have is in your mitochondria, mt-DNA. Mitochondria are small objects within your cell that are the power plants for your body. The mitochondria take the glucose in your blood and convert it into a form of energy useful to power all activities of the cell. Mitochondria are thought to have once been bacteria that formed an alliance with cells billions of years ago to work together. The mitochondria act as power plants and the cell provides the food. When a baby is conceived, the mother and father each contribute to the DNA in the cell nucleus, but the new baby gets all of its mitochondria from the mother. As a result, analysis of the DNA in the mitochondria allows one to determine the maternal line ancestry. SNP mutations also occur in the mt-DNA and scientists have developed a Haplotree for this DNA as well. Each person has both a Y-DNA Haplogroup and an mt-DNA Haplogroup. The mt-DNA Haplogroup allows you to determine the route your ancestors took in their migration out of Africa. In general, the mt-DNA is less useful than Y-DNA for two reasons. The rate of mutations is very slow so if two people have identical full sequences of their mt-DNA, their common ancestor could be thousands of years in the past. Also, most people find it difficult to track their maternal line using traditional genealogy due to the fact that maternal surnames change with each generation and are often not included in marriage records and wills.

DNA Structure

Points of Contact

To learn more about the Clan Henderson DNA Project contact the DNA Project Administrator:

David Henderson

To get personal assistance with DNA or Genealogy related issues, contact the Clan Henderson Society DNA and Genealogy Coordinator. She can answer simple questions and suggest one of over 30 volunteers who might provide assistance. Note: this is a free service, managed by volunteers. First, priority is given to members of the Society. Second, we ask that you have patience--we are not a "for profit" company.

Tracy Rowan DNA and Genealogy Coordinator