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STR-ing along

The Legal Genealogist is on the road, so the blog during this extended trip is featuring terms we all need to understand and use as genealogists.

Today’s term, for DNA Sunday, is STR.

It’s an acronym, short for Short Tandem Repeat, and it’s an important part of DNA analysis particularly for the YDNA direct-male-line testing so many of us and our kin are doing.

By definition, “A short tandem repeat (STR) in DNA occurs when a pattern of two or more nucleotides are repeated and the repeated sequences are directly adjacent to each other. An STR is also known as a microsatellite.”1

Hmmm… that’s not all that helpful, is it?

Try it this way: STRs are “short sequences of DNA, normally of length 2-5 base pairs, that are repeated numerous times in a head-tail manner.”2

Not clear, still?

How about this, from the Stewart Society’s genetic genealogy page: “STRs (short tandem repeats) measure the number of times a sequence of genetic code is repeated at a specific location on the Y-chromosome.”3

Still a little fuzzy? Think of it this way. All of our DNA — every single spot in every single location — is one of four chemicals (called nucleotides) — adenine, cytosine, guanine or thymine. Think of them as A, C, G, and T. When a lab computer reads our DNA, it’s detecting which of those four appears at a particular location.

An STR is when you get repeat sequences, say, GATC, again and again at that location. And how many of those repeat sequences helps us group people genetically, or separate them genetically, into families.


The chart above shows three participants in a hypothetical family YDNA project. One of the men has seven repeats, one has eight and one has nine. If all other markers are the same among them, then John is a genetic distance of one from David and two from Tom. David is a genetic distance of one from each of the others. Tom is a genetic distance of one from David and two from John.

Got it?

If not, just STR-ing along with me. We’ll get there eventually…


  1. ISOGG Wiki (, “Short tandem repeat,” rev. 20 July 2013.
  2. What is a Short Tandem Repeat Polymorphism (STR)?,” The Biology Project, University of Arizona ( : accessed 13 June 2015).
  3. “STR markers and SNP markers,” Bannockburn Genetic Genealogy Project, The Stewart Society ( : accessed 13 June 2015).
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