Definition of Heterosis:
The superiority of F1 hybrid over both its parents in terms of yield or some other characters or heterosis is increased vigours, growth, yield or function of a hybrid over the parents, resulting from crossing of genetically unlike organisms.
The term heterosis was first coined by Shull in 1914. Generally heterosis manifested as an increase in vigour, size, growth, yield or some other characteristics. But in some cases, hybrid may be inferior to the weaker parent this is also regarded as heterosis.
The superiority of F1 is estimated over average of the two parents (mid parent). This practise has found some acceptance particularly in the practical studies. However, in practical plant breeding the superiority of F1 over mid parent is of no use since it does not offer the hybrid any advantage over the better parent. Therefore, average heterosis is of little or no use to the plant breeder. More generally, heterosis is estimated over the superior parent such heterosis is referred as heterosis. The term heteroecism is not commonly used since most breeders regard this to be only case of heterosis and referred to as such i.e Heterosis.
However, the commercial usefulness of a hybrid would primarily opened on its performance in comparison to the best commercial variety. In many cases the superior parent may be inferior to the best commercial variety. In such cases, it will be desirable to estimate heterosis in relation to the best commercial variety is commonly known as economic or useful heterosis. Economic heterosis is the only estimate of heterosis, which is of commercial or practical value in 1944. Powers suggested that, the term heterosis should be used only when the hybrid is either superior or inferior to both the parents.