A number of groups around the world began to perform GWAS on measures derived from brain images, with the goal of finding new genetic variants that might account for more of the variation in brain structure and function, and also for disease risk. The genetic variants of interest in a GWAS are single nucleotide polymorphisms, or SNPs, commonly carried variants in the genetic code. SNPs are DNA sequence variations that occur when a single nucleotide (A, G, C, or T) is altered; SNPs are thought to be point mutations that were not so damaging that evolution allowed them to be retained in a significant proportion of the population of a species. Within a population, SNPs can be assigned a minor allele frequency—the proportion of chromosomes in the population carrying the less common variant. Figure 1 illustrates the ideas behind this approach. Before we discuss GWAS, it is worth noting a distinction between narrow and broad-sense heritability: broad-sense heritability is the proportion of variation in a phenotype (here, individual variations in brain measures) that can be explained by genetic effects. These
