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Learn about the comparison between inbreeding depression and hybrid vigour.
1. Increase in Homozygosity vs. Development of Heterozygosity:
Due to inbreeding each line becomes increasingly homozygous, as a consequence the variation within a line decrease rapidly. After 7-8 generations of selfing, the lines becomes almost uniform (99% homozygosity) which are called inbred lines.
Hybridization always favours hetrozysity; the species which reproduce by cross-fertilisation are heterozygous. Due to heterozygosity the effects of many recessive alleles are not expressed in heterosis, only the dominant effects or the multiple effects are expressed.
2. Appearance vs. No/Less Expression of Some Lethal and Sub-Lethal Alleles:
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Inbreeding may result in appearance of many harmful characters due to accumulation of harmful recessive alleles after selfing, e.g., chlorophyll deficiency (albina, chlorina), rootless seedlings, defective floral parts, etc. This type of effects is not found in case of heterosis as most of the lethal characters are expressed in homozygous condition. Heterosis or hybrid vigour prevails heterozygosity, so appearance of such characters does not happen.
3. Reduction vs. Increase in Vigour, Yield and Reproductive Ability:
Due to inbreeding there is a general reduction in vigour of the population, plants become shorter and weaker. The hybrids are generally more vigorous, healthier and increased in size. The reproductive ability also decreases in the population rapidly due to inbreeding, many lines reproduce so poorly that these cannot be maintained. The hybrids exhibiting heterosis show an increase in fertility or reproductive ability.
Inbreeding generally leads to loss in yield; the inbred lines yield much less than the open pollinated varieties from which they are derived. Heterosis is generally expressed as an increase in yield of the hybrid. Commercially this phenomenon is of great importance as an objective of plant breeding.
4. More Susceptible to Disease vs. Increase in Disease Resistance Property:
Due to inbreeding as homozygosity increases, there may be rapid loss of vigour as well as disease resistance property. Whereas due to heterosis the hybrids are known to exhibit a greater resistance to insects or diseases than the parents.
5. Less Adaptable to Changed Environment vs. Greater Adaptability in Hybrids:
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The inbred lines are homozygous, so they are less adaptable to changed environment as the modification of characters are not possible according to need. Whereas the hybrids are generally more adaptable to environmental changes than the inbreds. Variation due to heterozygosity offers the hybrids more adapted to environmental variations.
