MR Dictionary

Allele

A variant form of a single nucleotide polymorphism (SNP), a specific polymorphic site or a whole gene detectable at a locus. Most SNPs are biallelic, meaning that there are two values (forms) that are possible (e.g., the rs234 SNP denotes that individuals could have either an A or G allele at position 105,920,689 on chromosome 7 on the positive strand). At a given SNP, the most common allele is referred to as the ‘major allele’; whereas, the least common (or rarer) allele is referred to as the ‘minor allele’. The minor allele frequency is therefore the frequency at which the minor allele occurs within a population.

In MR, most commonly, the genetic instrumental variable (IV) is one or more SNP(s) that are robustly associated with the risk factor/exposure of interest. The number of 'trait-increasing' alleles is used analogously to the lifetime ‘dose’ of the risk factor. The commonest unit of the genetic IV-risk factor or genetic IV-outcome associations are ‘per-allele’ – i.e., assuming a linear additive change across the three genotype categories. See DNA.

Schematic representation of chromosomes, DNA and genetic variants in a diploid cell. From Hartwig et al., with permission.
Figure 1.1 - Schematic representation of chromosomes, DNA and genetic variants in a diploid cell. From Hartwig et al., with permission.

References

Other terms in 'Useful genetic terms ':