Last update = 29 May 2020
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The Y chromosome compared to the X chromosome. From cdn.zmescience.com
The body contains 3 types of DNA that can be tested, each of which gives a different part of an individual's ancestry. The surname projects at Family Tree DNA center on Y-chromosome DNA (YDNA) tests, and this page is about interpreting Y-DNA results. The other types are autosomal DNA (atDNA) and mitochondrial DNA (mtDNA).
Note that the tests sold by Ancestry and 23&Me are primarily atDNA tests, and as such cannot pinpoint family lines the way Y-DNA does.
Y-DNA tests are based on the Y-chromosome. All men have a Y-chromosome. What makes Y-DNA different from other types of DNA is that fathers pass this chromosome on to their sons, the same way they pass their surname down. The sons in turn pass their Y-chromosome (and surname) on to their sons, forever. Thus all men within a family share an identical Y-chromosome, except for rare changes that happen from time to time.Fortunately, DNA testing is simple and painless: just a few cells scraped from the inside of the cheek are all that is needed. See photos from the Dorsey DNA project.
Men who get their Y-DNA tested will get results that look like a string of numbers, such as 10-22-14-10-13-14-11-etc. In general, the closer the strings of numbers match, the closer a relationship is between two men. Men of the same immediate family share the the same string of numbers; closely related men have similar strings of numbers; unrelated men have different strings of numbers. The figure below shows what a typical result would look like on FTDNA. The top row is of DYS numbers, while the rows below each represent one person tested. Only two rows are shown.
Each of the numbers in the string represents what is called a short tandem repeat (STR). DNA is made up for 4 components, known as base pairs, and designated as C, G, A, and T. These occur in long strings (chromosomes) millions of base pairs long. Occasionally these base pairs occur as combinations that repeat themselves, ie, an STR:
Each STR is located on s specific place of the Y-chromosome; each place is indicated by a DYS number. Because each STR identifies s specific spot on the chromosome, it is called a marker. For example, at a location called DYS388, the sequence 'ATT' can be repeated 10 to 16 times. The DNA of a man who is DYS388=11 (i.e., 11 repeated copies of ATT at the DYS388 location) would look like this:
A hallmark of these STR markers is that they can increase or decrease in the number of repeated sequences from time to time:
It is this ability to change in copy number at the different DYS locations over hundreds of years that results in the string of numbers that is unique to each family.
This ability of STR markers to move up and down is also their largest limitation. For example, if DYS388 starts at 10, goes to 11, and then back down to 10 after several generations, it will appear as if it never changed, and give impression that two families are more closely related to each other than they really are. So while STR markers are useful to determine family relationships over some 10 generations or so, they are not as good over longer periods of time. This limitation is overcome by SNP markers, which are presented next:SNP markers
A second type of DNA change that also serves as a marker is known as a SNP (single nucleotide polymorphism). It simply involves the change of 1 base into another, as follows:
The following example illustrates how SNPs are used for genealogy by finding a branch within a larger tree. Essentially, SNPs are like mileposts that are laid down consecutively along the tree trunk as it grows. The same happens for each branch of the tree.
In the illustration below, SNPs are represented as gray markers.
First, the SNPs along the main trunk get tested, from the root upwards, until they start turning turning up negative. In this example, positive SNPs are red, negatives are in yellow. The branch being researched will be between the last positive SNP and the first negative SNP.
Finally, the process is repeated on the branch identified from the previous step. The proper sub-branch is shown here by a bull's eye.
In practice, SNPs are rare-- they happen only once per about billion base pairs in any generation, so they can be considered pretty much permanent changes. The date of their appearance can be timed, allowing them to be arranged in chronological order in a family tree. They are therefore useful to look at family relationships that are a few centuries old, before surnames became established. Any person will either have a SNP or they won't. Once a SNP appears in the Y-DNA of a family, all male descendants will have it. Thus individuals are classified as being positive or negative for any given SNP, and branches can be arranged chronologically based on the presence or absence of a given SNP.
For example, if all individuals in a group are positive for a SNP called L338, but only some for S12289, that means that L388 is older than S12289. S1990 is another SNP in group. Everyone who has S1990 also has S12289, but not the other way around. Thus S12289 is older than S1990, and so forth down the line.Haplogroups
Y chromosomes from different geographic areas or ethnic groups (known as haplogroups) are recognizable by their combination of markers. In recent years, the main haplogroupos have come to be defined by SNP markers at their base (where they branch off from the main tree) whereas their branches have come to be named by the last SNP at their tips. See the current ISOGG or YFull tree for the latest classificaton of haplogroups.