Heterosis:
Hybrids between two different races or varieties of animals and plants exhibit a general increase in size, strength, stamina, and vigour. This quality of hybrids is called hybrid vigor or heterosis.
Early plant hybridizers had noticed that when cross-fertilization is effected between two pure strains or races, differing in a number of characters, the resultant hybrids were superior to either of the parents with regard to a number of morphological and physiological features.
The general increase was noticed in size, vigour, vitality, productivity, increased resistance to disease, more adaptability to the environment, etc. The increase in the vigour of hybrids is not a permanent feature. If crosses are repeated the vigour is maintained. But if these are not crossed further the vigour decreases in successive generations.
The experiments on cross-breeding were conducted by a number of scientists like Knight, Mendel, Darwin, Herber, Beal and Shull (1910) proposed the term heterosis for this superiority in hybrids.
Genetic Basis of Heterosis:
The exact cause for the appearance of heterosis is still not properly understood.
1. Physiological stimulus or Heterozygosity: According to shull (1910) heterosis is due to some kind of physiological stimulus produced by the union of dissimilar gametes from two separate races.
2. Dominant factor hypothesis: Bruce suggested that superiority is governed by a number of genes. Due of the two pure lines brings together these dominant genes in the hybrid. Bruce calculated that when the dominant genes of one parent combine with those of the other one. There is an increase in the total number of dominant genes in the zygote. This is the most widely accepted hypothesis.
3. Cytoplasmic nuclear reaction hypothesis: According to Michaelis, Shull and Lewis heterosis is due to the interaction between nuclear and cytoplasmic systems.
Significance of Heterosis:
Hybrid vigour has practical importance. It has been experimentally introduced in a large number of plants and animals. In plants, hybrid vigour is expressed in the form of height, viability, a greater yield of fruits and seeds, the large size of fruits and seeds, increased resistance to environmental factors, resistance to decrease pests and better seed germination, etc. It has been seen in tomatoes, onions, grasses, tobacco, cotton, sugarcane, brinjal, cucumber and several other ornamental plants.
Among animals, hybrid vigour is expressed in the form of milk production in cattle, an increase in the number of eggs, better pork and beef output and being sturdier than either of his parents. Similarly, hinney is a hybrid of horse and Janette and zebronsky is produced by a cross between zebra and donkey.