Humans share 99.8% of the same identity at the genetic sequence level, suggesting that the variation in individuals' characteristics, i.e., phenotype, stems from the remaining 0.2%. Differences in DNA sequence arise due to the presence of genetic variants, called single nucleotide polymorphisms (SNPs). This DNA variation is the substitution of one DNA base for another in a genetic locus and comprises the most common type of genetic polymorphisms.

Almost half of the SNPs in the human genome can affect gene expression and function, which in turn can lead to phenotypic variation between individuals, such as personalised responses to food and exercise and predisposition to common diet-related diseases such as obesity, metabolic syndrome type 2 diabetes, as well as cardiovascular disease.