DEFINITION

• A single nucleotide polymorphism (SNP) is a variation in a single nucleotide of the DNA that occurs at a specific position within the genome. SNPs can be easily genotyped and are the main source of genetic variation studied in genetic association studies.
• Polygenic refers to the cumulative effects of many genes, each having very small effects.
• A polygenic risk score (PRS) is a quantitative score that characterizes an individual’s genetic risk of disease. The PRS is generated by summing the number of risk alleles for that individual across multiple loci. For a PRS based on 100 loci, an individual’s PRS would thus range from 0 to 200, depending on whether the individual had 0, 1, or 2 copies of the risk allele at each locus. Each risk allele can also be weighted by its effect size.
• Epigenetic effects involve changes to a phenotype that arise from changes in gene expression from mechanisms other than modification of the DNA sequence itself. The three major mechanisms contributing to epigenetic changes to gene regulation are DNA methylation, histone modification, and RNA regulation.