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What is STR and SNP DNA?

Alasdair Macdonald and Graham Holton explain the difference between STR DNA and SNP DNA in understanding your family history

STR and SNP DNA

Y-DNA testing is used to research male ancestry – but there are two kinds of Y-DNA test, SNP DNA and STR DNA, which have different uses and purposes in DNA testing.

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What is STR DNA?

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 STR DNA 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. The STR DNA test is recommended for establishing male-line relatedness in the past 400–500 years.

What are the pros and cons of an STR DNA test?

The pros of an STR DNA test are that it can be used to find your genetic family surname, and may suggest what haplogroup and subgroup you belong to. The cons are the unstable markers, and the fact that it cannot define precisely when your most recent common ancestor with your matches lived.

What is SNP DNA?

A single nucleotide polymorphism (SNP) is a mutation in one of the nucleotide bases that are the building blocks of our DNA code. Unlike STR DNA, SNP DNA is 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 SNP DNA testing particularly useful for distinguishing one genetic lineage from another.

SNP DNA 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.

What are the pros and cons of an SNP DNA test?

The pros of an SNP DNA test are the stable markers, and the increased accuracy in identifying your most recent common ancestor with your distant family. The cons are that SNP DNA testing is more expensive than STR DNA 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 nobles