The unusual situation where a heterozygote shows an attribute, such as viability or fertility, that is lower than either homozygote. For example, the New Zealand Black (NZB) strain of mouse spontaneously develops a disease that resembles lupus erythematosis in humans (q.v.). New Zealand White (NZW) mice are normal in this regard. The hybrid offspring from crossing these inbred strains of mice (NZB × NZW) develop a more severe disease than that of the NZB strain.
Subjects: Genetics and Genomics.