7. how can differences between individuals such us as SNP\'s lead to persistent expression of lactase is some people but not in others? Solution Lactase persistence is the continued activity of the enzyme lactase in adulthood, only function of this enzyme is the digestion of lactose in milk, in most mammal species, the activity of the enzyme is dramatically reduced after weaning. Two mutations (single-nucleotide polymorphisms – SNPs) have been associated with lactase expression. C-14010, G-13907, and G-13915, T13910 and A22018 are related to lactase persistence.Schultheis et.al. suggests this trait is strongly associated with a C to T substitution at a polymorphic site 13,910 bp upstream of the lactase gene. As per Swallow DM\'s paper stated that , \"A putative causal nucleotide change has been identified and occurs on the background of a very extended haplotype that is frequent in Northern Europeans, where lactase persistence is frequent. This single nucleotide polymorphism is located 14 kb upstream from the start of transcription of lactase in an intron of the adjacent gene MCM6.\" So if the single nucleotide polymorphism occurs then the lactase persistance is seen in some individuals else not..