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Understanding Y-STRs

A Y-DNA Short Tandem Repeat (STR) is a unique sequence of nucleotides that repeats itself multiple times within a continuous stretch of the Y chromosome. We call these sequences markers. The number of times this sequence repeats is called the value of that STR marker. Each marker is designated with a unique identifier and is in a fixed location on the chromosome. 


When STR markers are identified, they are assigned an alpha-numeric designation by the lab that first identified them. This can be done independently by different labs, and for this reason, the same marker might be known by different names. Markers identified in the early days of testing often had different names, which led to confusion in locations and values. 

Today, STRs are named according to guidelines published by the HUGO Gene Nomenclature Committee. However, when new STRs are discovered, sometimes placeholder names are applied until they receive an official designation by HUGO. This has been the case with STRs discovered by the Big Y test offered by FamilyTreeDNA.

 STR Tests offered by FamilyTreeDNA are labeled with the following prefixes:

  • DYS, DYZ, and DYF
    • D stands for DNA.
    • Y stands for Y chromosome.
    • S, Z, and F stand for the complexity of the repeat segment as follows:
      • S is a unique segment.
      • Z is a number of repetitive segments at one site.
      • F is a segment that has multiple copies on the Y chromosome.
  • FTY - This stands for Family Tree Y. For now, this prefix acts as a placeholder until HUGO assigns an official prefix to these STRs.


The Genome Research Consortium periodically releases a set of coordinates for all known nucleotides in the human genome based on the latest research in the field. This is called a build and serves as a reference sequence that is used throughout the genetic community as a standard to identify markers and mutations. 

The current build used by FamilyTreeDNA (and the community in general) is the Genome Research Consortium human build 38, abbreviated as GRCh38. This is commonly nicknamed hg38 for human genome build 38.

Each STR marker, regardless of its name, has a specific standard location in hg38. For example, the sequence designated DYS19 by HUGO is also known as DYS394, but both names refer to a repeat of the sequence TAGA between locations 9,684,325 and 9,684,519 in build hg38.

For more information on the details of how these sequences are identified and isolated, see the DNA Structure and the Testing Process topic.

Measuring STRs

Because of the repeating nature of STRs, a repeat can be gained or lost from one generation to the next without affecting the normal functioning of the cell. This usually happens when new copies of DNA are created. 

For example, in DYS19, a father may have 16 repeats of nucleotide sequence TAGA between locations 9,684,325 and 9,684,519. In the creation of a new embryo, it is possible that one of these repeats could be gained or lost, resulting in 15 or 17 repeats. The number of repeats a person has for an STR is called their STR value. In this example, we would say the father has DYS19=16, while the son has DYS19=15 or DYS19=17 depending on whether a repeat was gained or lost.

The number of repeats within an STR does not change the location of either endpoint. In our example, each endpoint of DYS19 will still be identified as 9,684,325 and 9,684,519 in build hg38 regardless of whether it has 15, 16, or 17 repeats. This is due to the fact that those endpoints are not affected by the number of repeats between them..

In some cases, the same STR can occur in separate locations. Even though the location is different, they will still display the same character sequence of nucleotides, such as TAGA. When this happens, we call them multi-copy markers.

For more information on multi-copy markers, see the Understanding Y-DNA Multi-Copy Markers topic.

Application to Genealogy

Changes in STR values do not occur in every generation. When they do, the changed value is usually passed down to subsequent generations. This means that as two different lineages diverge from a common ancestor, the changes in STR values tend to accumulate. Generally speaking, the greater the number of differences in STR values between two people, the further removed in time their common paternal ancestor lived.

Differences in STR values can help identify patterns in patrilineal inheritance. When we compare the STR values of two different people, we measure it with Genetic Distance (GD). 

For an in-depth look at Genetic Distance and how it is calculated, see the Understanding Y-DNA Genetic Distance topic.

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