Causes of superiority
Case I: According to Shull (1910), heterosis is due to some kind of physiological stimulus produced by union of two dissimilar gametes from two separate genes. Hybrid substance is superior over the parents because combination of dominant parents resulting in superiority due to chemical substance releasement.
Case II: AB blood group is due to presence of both A and B antigen in F1 leading to superiority of F1 over parents.
Case III: Alkaline phosphatase mutations in E.coli , mutant polypeptides produced by two different alleles may complement each other within a single multichained protein produce a more effective product. Due to complementation of A and a, alkaline phosphatase is formed leading to superiority over the parents.
Case IV: In corn, Schwartz and laughner demonstrated that a hybrid possess an alcohol dehydrogenase enzyme that is both active and stable compared to inactive or unstable parental enzymes. Aa forms Alcohol dehydrogenase enzyme, which is active and stable but AA and aa produce those enzyme which are not active and stable leads to superiority over parents.
Case V: If each homozygote produces a different metabolic substance, the heterozygote may produce both substances and thereby advantages may arise from the simultaneous possession of two different metabolites. This can be seen in sickle cell heterozygote who possess both sickle hemoglobin that is believed to prevent malarial infection and normal hemoglobin that prevents hemolytic anemia.