It is in the copying of DNA, in the production of sperm and eggs that changes,variants or markers arise. If you were asked to copy six billion letters, think how many mistakes you would make?! The ‘molecular machine’ that copies our DNA is much more reliable, but not infallible, and it is these changes which make us different and show that we descend from different lineages.
These markers, or Single-Nucleotide Polymorphisms (SNPs - pronounced snips) can be found to originate in particular parts of the world, and through a measurement called the molecular clock they can be dated. The molecular clock hypothesis suggests that there is a roughly constant rate at which the ‘molecular machine’ makes these errors - approximately once every generation or two on the Y chromosome.
If the 6,000,000,000 letters of a single persons DNA were notes in a song, played at an allegro tempo - 120 beats per minute - it would take almost a century to play! We extract DNA from your saliva and read your genetic code at a number of variable points in your sequence of letters in order to determine your personal DNA signature.