Tracking the Y-Chromosome

Single Nucleotide Polymorphisms (SNPs) are binary markers that can be identified in the DNA of the Y chromosome; they occur very rarely, so that two men sharing a particular SNP state in their DNA almost definitely inherited this from a shared ancestor, who may have lived many thousands of years ago. Setts of SNPs are typed, and used to define Y chromosome types called haplogroups, which in turn can be arranged into a 'family tree', or phylogeny. If two men belong to different haplogroups, this excludes them from sharing a recent common ancestor. An example of a SNP is shown in the diagram below.

Short Tandem Repeats (STRs)
Another type of marker on the Y chromosome consists of short units of DNA (typically 3 or 4 bases long) that are repeated in tandem several times (see diagram above). These are called microsatellites, or short tandem repeats (STRs); variation in these markers occurs much more frequently than at SNPs. Different Y chromosomes can carry varying numbers of repeats at a set of specific STRs, called a Y-STR haplotype. Haplotypes can be compared to identify differences between men within the same haplogroup. Related men will show the same or very similar patterns in terms of the numbers of repeats seen at a set of markers, while unrelated men will tend not to.
- Surnames, Genes and the History of Britain