What is STR and SNP DNA?

Alasdair Macdonald and Graham Holton explain the difference between the two types of Y-DNA test in genetic genealogy

STR and SNP DNA

Y-DNA testing is used to research male ancestry – but these two types of DNA test are both important tools in genetic genealogy.

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STRs

Short tandem repeats (STRs) measure the number of times a sequence of genetic code is repeated at a specific location on the Y-chromosome. The common tests available are on 37 or 111 markers. Since the values for these markers change over time, they are useful for establishing the timeframe when two men shared a common paternal ancestor. This is the type of test that is recommended for establishing male-line relatedness in the past 400–500 years.

Pros

  • Can often be indicative of your genetic family surname
  • May suggest what haplogroup and subgroup you belong to

Cons

  • Unstable markers
  • Cannot define precisely when your most recent common ancestor with your matches lived

SNPs

A single nucleotide polymorphism (SNP) is a mutation in one of the nucleotide bases that are the building blocks of our DNA code. Unlike STRs, SNPs are very stable over many generations. When a mutation does occur, it is carried indefinitely by descendants of the individual in whom the SNP was formed – the ‘SNP Progenitor’. This makes them particularly useful for distinguishing one genetic lineage from another.

SNP tests are available as single SNP tests, SNP packs or ‘next-generation sequencing’ (NGS) testing such as FamilyTreeDNA’s Big Y-700 (which doesn’t come cheap at $399). SNP packs only test a limited number of SNPs, and in most cases they will be SNPs that were not formed recently.

Pros

  • Stable markers
  • Can usually define more accurately than STRs when your most recent common ancestor with your matches lived

Cons

  • Require additional expense beyond STR testing
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Read more from Alasdair Macdonald and Graham Holton on how they used STR and SNP DNA testing to discover the modern-day descendants of a group of 14th century Scottish rebels