Metrics or operations on genome sequences in human genomic research?
The question of how similar two DNA or protein sequences are, is one of the most important problems in bioinformatics. While there is no definitive answer Bayseian statistics are often used to group sequences. There is a good explanation of the theory on this wikipedia page. 

Alternatively one of the most commonly used programs to find similar sequences is BLAST. It is getting a little bit dated but you can find an overview on the statistics used by this program on this page

But there are many other methods out there.