Inbreeding Depression and Heterosis

In most populations of animals, or higher plants, mobility is restricted. Consequently, mating between individuals of such populations is far from random, this being affected among individuals occurring in proximity with each other in space. Many small sub-gene pools are thus formed within a gene pool. These sub-gene pools depart to some extent, from overall characteristics of the entire pool. One of the consequences of restriction in population size due to restricted mobilities is mating between relatives, which is also called 'inbreeding'. Inbreeding, though, has very little influence on change in overall gene frequencies, has major effects on frequency of homozygotes, which is increased. If a recessive gene is rare, it will appear in homozygous condition at a higher frequency under inbreeding than under random mating. As a result, selection will be provided with increased opportunity to act upon rare recessives. Inbreeding, however, has been associated with unfavourable biological effects, and crossing with an unrelated stock results in increased vigour. This may perhaps be the reason why consanguineous marriages, (marriages between close relatives) have been forbidden. The consequences of inbreeding may, therefore, have wide implications for both natural and artificial selections.

The advantage of cross-breeding in plants was known as early as the 18th century. J. Kolreuter in 1763, Sprengel in 1793 and Charles Darwin in 1876 recorded the advantages of cross-pollination and concluded that even nature favoured cross hybridization for survival of a species. Numerous structural and functional adaptations were recorded which helped plants with hermaphrodite flowers avoid self-fertilization. Darwin concluded that self-fertilization was an unnatural and harmful process. These observations were not fully understood at that time. However, they later made the basis of much breeding work on cross pollinated crops. Following generalizations can be made with respect to inbreeding and hybrid vigour : (i) inbreeding often leads to loss of vigour and to other indications of deterioration, (ii) hybridization between unlike types is often accompanied with great vigour, and (iii) crossbreeding must be important biologically. Before we discuss any details of cross-breeding and its manifestations, it would be desirable to have some idea about observations on inbreeding depression and heterosis.