Missing Heritability and Missing Co-heritability in Genomic Studies

In my article, Scientific and Technological Interventions for Attaining Precision in Plant Genetics and Breeding, published on March 30 2018 7, I had reviewed the development since Mendel's discovery of genetic laws, in terms of cloning technology and reverse genetics, chip technology, genetically modified organisms and CRISPR-based gene editing technology, in particular their roles in further refining plant genetics and breeding practices. It is well known how statistics entered in genetics due to work of Fisher 2 when he decomposed the phenotypic variability into variability due to additive effects of genes and that due to environmental effects paving the way for the role of heritability (proportion of the former due to that by additive effects of genes) with symbol h2. Soon after the introduction of chip technology – the genotyping of molecular markers – the method of plant breeding got a big impetus in increasing precision in the breeding process by incorporating the marker information in the methods of selection and cross breeding. The position of markers in the genome is fully known through linkage maps. With this information the methods of quantitative genetics got modified in that the variability due to additive effects of genes got further decomposed into that due to association of marker information with the genes controlling the trait and the rest. The proportion of additive genetic variability due to such association gives rise to another parameter p in addition to h2. The expression ph2 then refers to the proportion of phenotypic variation due to marker/markers information in the genome. This fraction becomes just h2when markers are totally linked with the genes controlling the trait. But when the markers are not at all associated with the genes for the trait, p becoming zero, this becomes zero. The difference (h2– ph2) is then referred to as missing heritability. In several genome-wide association studies (GWAS), genes have been identified associated with diseases but they explain only a small fraction of the variability leading to the frequently asked question of missing heritability4. For most of the economic traits in plants and animals, heritability is usually small, making missing heritability inevitable.