Two different types of marker are used in Y-DNA analysis. Their names may sound complicated but can be quite simply explained if we think of DNA as a code written in four letters: A, C, G & T.
A “short tandem repeat” (STR) is a small section of repeated code e.g. GATA,GATA,GATA etc.
The number of repeats are counted and may be compared between people.
A “single nucleotide polymorphism” (SNP) is a point mutation and represents a position on the chromosome where a single letter of code has changed e.g. a “G” has changed to an “A”.
Surname projects were founded on Y-STRs and these results form the bulk of their data. These are the numbers visible in the results tables.
The problem with STRs is that they can go up or down at random and it is often not possible to tell the order in which changes occurred. Co-incidental changes and “back-mutations” (where a number changes back to an earlier state) can confound the interpretation of results.
SNPs, by contrast, tend to be “one-off” events. They still occur at random but we can generally tell the order in which they occurred. They can be very helpful for identifying individual family lines.
SNP analysis has only really become readily available for genealogy within the past few years.
Genetic genealogy projects presently use a combination of STR and SNP analysis, to great effect. The interpretation of results must be done within context and this is where the role of the project administrator becomes crucial.
Anyone considering DNA testing should carefully consider their aims and then seek advice from a suitable expert on which DNA test to take. The choice is extensive and it is vital to take time and make the right decision.
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