Principles of Plant Breeding (2+1)

Heterosis and Hybrid Vigour

• But a vast majority of the cases of heterosis are cases of superiority of hybrids over their parents. Hybrid vigour describes only superiority of hybrids over the parents. The few cases where F1 hybrids are inferior to their parents may also be regarded as cases of hybrid vigour in the negative direction.

Heterosis is the superiority of a hybrid over its parents.
1. Increased yield: Heterosis is generally expressed as an increase in the yield of hybrids. The yield may be measured in terms of grain, fruit, seed, leaf, tubers or the whole plant.
2. Increased Reproductive Ability: More number of flowers/fruits/seeds.. Increase in Size and General Vigour: The hybrids are generally more vigorous, i.e., healthier and faster growing and larger in size than their parents. .
3. Better Quality: In many cases, hybrids show improved quality. For example, many hybrids in onion show better keeping quality, but not yield, than open-pollinated varieties.
4. Earlier Flowering and Maturity: In many cases hybrids are earlier in flowering and maturity than the parents. But earliness is highly desirable in many situations particularly in vegetables.
5. Greater resistance to disease and pest: Some hybrids are known to exhibit a greater resistance to insect or diseases than their parents.
6. Greater adoptability: Hybrids are generally more adapted to environmental changes than inbreds..
7. Faster growth rate: In some cases, hybrids show a faster growth rate than their parents. But the total plant size of the hybrids may be comparable to that of parents. In such cases, a faster growth rate is not associated with a larger size.
8. Increase in the number of a plant part: In some cases there is an increase in the number of nodes, leaves and other plant parts, but the total plant size may not be larger.

Main Difference between Inbreeding and Heterosis:

Inbreeding	Heterosis
1. Inbreeding results from matings between closely related individuals.	Heterosis results from crossing between unrelated strains.
2. It is decline in fitness and vigour with decreased heterozygous.	In heterosis the unfavourable recessive genes of one line (parent) are covered by favourable dominant genes of other parent.
3. It results due to fixation of unfavourable recessive genes in F2.	In this case unfavourable recessive genes of one lien (parent) are covered by favourable dominant genes of other parent.