You could look at a genome as a genetic code written using an ‘alphabet’ of just four chemicals – A (adenine), C (cytosine), G (guanine) and T (thymine) – and if you do that, then each genome is made up of about three billion individual letters.3 A 0.1 per cent difference between the genomes of two people translates to about three million differences between the two genomes. If the two genomes came from people who shared a recent ancestor, then the differences would be smaller (which also means that genetic differences can be used as a measure of how close or distant two individuals are
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