Genome-wide association studies (GWAS) have been successful at discovering common genetic variants that are risk factors for disease or that influence traits. Although more than 1,000 genetic loci, or regions of the genome, have been associated with diseases or traits through GWAS, much of the heritability remains hidden. A disease or trait’s heritability is the degree to which it is inherited, and therefore, influenced by genetic elements; scientists often determine heritability through studies of siblings and twins to tease out genetic and environmental effects.