The Y-chromosome has been passed on from father to son since the beginning of mankind.  Spontaneous changes (mutations) to single ‘letters’ in DNA occur from time to time and result in what are called SNPs (single nucleotide polymorphisms, pronounced ‘snips’).  Each time a mutation occurs, this is a new SNP that will passed on to all male descendants.  These changes occur infrequently, but have built up over the millennia, and the SNPs on every man’s Y chromosome record the history of these changes.  By comparing the SNPs on many Y chromosomes, a high-level family tree for mankind has been constructed which is called the Y-chromosome haplotree.  The end of each branch or twig on the tree is a called a haplogroup.  The most detailed Y-DNA testing seeks to determine which haplogroup a person is in. But because only a small percentage of people have been tested, the haplotree is incomplete.  As new tests are done, new twigs are added to the tree.

For a surname-specific DNA study, we have at least some information about our own family trees and we may suspect that we might be related to other families with the same surname.  Y-DNA testing helps by defining the haplogroups for each family, and enabling us to understand which families are related, and how far back in time the common ancestor lived.

A wrinkle to be aware of is that sometimes there is a so called ‘non-paternity event’ (NPE).  This means that we may know that two people are close cousins, for example, but if their Y-chromosomes are very different then it means that somewhere in the tree the presumed father is not actually the biological father.  Adoption would have this effect, for example.  Over a few generations this is unlikely to occur, but over many centuries it is increasingly likely. 

There are two types of Y-DNA tests.  Y-STR tests and Y-SNP tests.  Y-STRs (short tandem repeats) are small lengths of DNA that are repeated several times next to each other on the Y-chromosome.  A 37 STR test looks at 37 places on the Y-chromosome DNA where STRs occur and counts the number of repeats found in each place.  The number of repeats in each place can change over time down the generations.  These changes happen more frequently than changes to SNPs.  This means that by comparing the STR tests of two people we can see how likely they are to be related to each other, and how closely, over the last 400 or so years.  Different tests on offer count the number of repeats at different numbers of places on the Y-chromosome – so a 37 STR test gives some level of information, whilst a 111 STR test gives more precision. 

The Y-STR tests are useful. However, haplogroups on mankind’s family tree (the haplotree) are defined by SNPs, not by STRs.  You can often predict what branch of the haplotree someone is on from an STR test, but you can only know for sure and pin it down to one small twig on the tree by doing a Y-SNP test, like the Big-Y test from FTDNA.