A genetic variant is a heritable difference in a DNA sequence relative to a chosen reference. Genetic variants can be used as instrumental variables in Mendelian randomization when the required causal assumptions are credible.
A single-nucleotide polymorphism is variation at one nucleotide position in sampled genomes. Under an infinite sites mutation model, the number of such variable positions in a sample equals the number of mutations on the genealogy below its most recent common ancestor.
A tag SNP represents other nearby single-nucleotide polymorphisms whose alleles are strongly correlated through linkage disequilibrium. Tagging reduces the number of markers needed for association mapping; it does not establish that the tag itself causes the phenotype.
// Target: biology.bigb
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