Genes are long chains of repeating sub-units called nucleotides, which come in the 4 forms represented by the letters A, T, C and G.
Each gene can be made of hundreds to thousands of nucleotides, and the sequence the nucleotides are arranged in determines the gene’s function.
On average humans are 99.5% genetically identical with each other, meaning the majority of these nucleotides will be the same between people.
However, at some key positions in the sequence, the nucleotides can come in a number of different forms, known as a polymorphism. When this occurs at one specific position, it is known as a single nucleotide polymorphism, or SNP.
These small changes can cause significant differences in how the gene functions.