overdominance

This term is a specialized technical expression used almost exclusively within the fields of genetics and evolutionary biology. It describes a specific state of heterozygote advantage, where the hybrid offspring possesses a survival or reproductive edge over both pure-breeding parents. This concept is central to explaining why certain deleterious alleles, such as those causing sickle cell anemia, persist in a population despite being harmful in a homozygous state.

Because it refers to a specific biological mechanism or phenomenon, the word is treated as an uncountable noun. It does not refer to a countable object but rather to a genetic principle or a state of being. It is rarely used outside of academic, scientific, or medical discourse.