single nucleotide polymorphism (SNP) is a change in a single nucleotide (i.e A, T, C or G) at a certain position in the genome. These SNPs may differ within an individual (e.g. two copies of a gene, one from mum and one from dad, with differences between them), from individual to individual or from ecotype to ecotype. SNPs can also be responsible for some diseases, e.g. sickle-cell anemia or thalassemia.