Abstract
Here we review the historical background and contemporary insights into genetic dominance, focusing on haploinsufficiency (HI), that is, when the function of only one allele of a gene is not enough to ensure a normal phenotype in a diploid organism. A related phenomenon is triplosensitivity, that is, pathogenic effects when there are three instead of two copies of some 'genes'. The importance of gene dosage issues was realized in humans when whole chromosomal abnormalities (aneuploidy) could be linked to clinical phenotypes such as Down, Edwards, and Patau syndromes. Subsequently, subtler chromosomal deletions and duplications have been shown to be responsible for many developmental syndromes. In several cases, a dosage-sensitive gene mapping to the relevant regions has been implicated as causal. We delve into the mechanisms of HI, especially due to direct protein insufficiency and subunit imbalances in the context of multi-subunit complexes. We show how the nonlinearity inherent to the relationship between genotype and phenotype is responsible for the dominance of the underlying genetic variants. We also explore why increased gene dosage can lead to abnormal phenotypes. Examples include trisomy or segmental genomic duplications in humans and oncogene amplification in cancers. Finally, we examine a few cases of genetic synergy, where the combined effect of two or more variants amplifies their individual effects, underlying a distinguishable phenotype. Further research is required to elucidate the dynamics of multicomponent interactions to unravel the mechanistic complexities of genetic dominance, inter-gene interactions, and their implications for disease.