Inbreeding Depression: Concept, Vipera Berus

Inbreeding is the form of a mating system in the sexual organism. It implies mating together of individuals that are close to each other by ancestral or pedigree relationship. There are two types of inbreeding:

1) Close inbreeding: Mating is done between highly closely related individuals in this sort of inbreeding, such as full brothers crossing with full sisters or offspring crossing with parents.

2) Line inbreeding: It is repeated backcrossing to one exceptional ancestor in order to increase its contribution to the progeny. Matings are made in this method of breeding to concentrate the inheritance of desired characters in a few preferred individuals.

a) It ensures that the required type is uniform.

b) The risks associated with breeding can be decreased.

Charles Darwin was the first to identify and quantify the negative effects of inbreeding, conducting experiments on 57 plant species that involved self-fertilization and outcrossing between unrelated individuals . Reduced survival and fertility of related individuals happen in wild animals and plant populations as well as in humans, demonstrating the natural populations have genetic variation in fitness attributes.

The impact of inbreeding is the loss of vigour and the physiological efficiency of an organism characterized by a reduction in size and fecundity.

Key takeaways: Inbreeding depression, plant depression breeding, animal depression breeding, homozygosity, heterosis, hybrid vigour, luxuriance.

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Inbreeding Depression

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Inbreeding depression results due to fixation of unfavorable recessive genes in F₂, while in heterosis the unfavorable recessive genes of one line (parent) are covered by favorable dominant genes of other parents.

The formula for inbreeding depression is:

Inbreeding depression =F₁ –F₂/F₁x100

(Where, F₁=first filial generation, F₂=second filial generation)

Inbreeding can be done in plants and animals.

Inbreeding Depression

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Plant Inbreeding Depression

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Based on the degree of depression, the plant species can be grouped into 4 broad categories:

1. High inbreeding depression: Inbreeding causes severe depression with lethal effects. After 3 or 4 generations of selfing, it is tough to maintain the lines. E.g: Lucerne, carrot
2. Moderate inbreeding depression: Though fatal effects are there, still lines can be segregated and maintained. E.g. Jowar, Maize, Bajra.
3. Low inbreeding depression: Only a small degree of breeding depression is observed .E.g.: Sunflower, Cucurbits.
4. No inbreeding depression: The self-pollinated grains do not exhibit inbreeding depression.

Figure: Plant inbreeding depression

Effects on inbreeding depression:

1. The appearance of lethal and sublethal alleles. E.g.: sub vital character.
2. Reduction in vigour.
3. Reduction in reproductive ability
4. Separation of the population into distinct lines.
5. Reduction in yield.
6. Increase in homozygosity.

Difference between high inbreeding depression, moderate inbreeding depression, low inbreeding depression.

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Heterosis

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Heterosis is the superiority leg yield /or inferiority of F1 over both the parents.

Types of Heterosis:

1. Average Heterosis: It is the heterosis where F1 is superior to mid parent value. In other words superior to the average of two parents. This heterosis plays no role in agriculture as superiority is below better parent value.
2. Heterobeltiosis: Superiority of F1 over comparatively better parent.
3. Useful/Economic heterosis: Superiority of the F1 compound to the high-yielding commercial variety in a particular crop.
4. Negative heterosis: Performance of F1 is inferior than better parent/mid parent value . E.g. Duration.

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Hybrid Vigour

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It is a synonym of heterosis, describing only the superiority of F1 over both the parents. Koelreuter (1673) is the first to report hybrid vigour in artificial tobacco.

Luxuriance:

Increased vigour and size of interspecific hybrids, produce sterile or poorly sterile plants.

Manifestation of heterosis:

1. Increased yield
2. Increased reproductive ability(e.g.: potato, sugarcane)
3. Increase in size and vigour.
4. Better quality
5. Early flowering and maturity(E.g: Tomato)
6. Resistance to pest and disease
7. Greater adaptability
8. Faster growth rate
9. Increase in a number of plant parts.

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Animal Breeding

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Animal breeding is a crucial aspect of animal care. Its goal is to boost animal yields and improve desirable attributes of food. Inbreeding occurs when closely related individuals within the same breed mate for a period of 4-6 generations. The following are some inbreeding strategies:

• Superior males and females of the same breed can be identified. They mate in groups of two.
• Evaluation of the progeny produced by the above marriage in order to find superior males and females.
• In the case of cattle, a superior female is a cow or buffalo that produces more milk per lactation. Bulls are superior men who produce superior progeny when compared to other males.

Inbreeding has the following effects:

• It is employed in the development of a pure line (inbreeding increases homozygosity).
• Inbreeding exposes harmful recessive genes, which are eventually eliminated by selection.
• The desirable genes can be acquired by inbreeding, whereas the superior genes can be accumulated through outbreeding.
• At every step, selection increases the productivity of the inbred population.
• Inbreeding has a negative impact on fertility and even production. Inbreeding depression is the term for this.

In this situation, animals from the breeding population are mated with superior animals of the same breed who are unrelated. This aids in the restoration of fertility and yield.

Figure: Inbreeding in animals.

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Advantages of Inbreeding

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• Further testing in this line may reveal and delete undesirable recessive genes.
• It boosts homozygosity while lowering genetic variation.
• The population is divided into distinct inbreed lines.

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Disadvantages of Inbreeding

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• Disease susceptibility increases in the progeny.
• In most cases, breeding issues and reproductive failure rise.
• It's difficult to pinpoint the point at which breeding should be stopped in order to avoid the system's negative consequences.
• It lowers vitality earlier in life than later in life.
• A tiny breeder has a strong possibility of profiting from excessive breeding.
• Because of its multiple negative impacts, inbreeding appears to have limited utility in dairy cattle breeding programs.

Also Read:

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Things to Remember

• Inbreeding depression describes a drop in average individual fitness in small populations caused by related individuals mating, resulting in the manifestation of recessive traits and an increase in genetic load.
• A population bottleneck is frequently the cause of inbreeding depression. In general, the greater the genetic diversity or gene pool within a breeding population, the lower the risk of inbreeding depression.
• Heterosis is the superiority leg yield /or inferiority of F1 over both the parents.
• A large proportion of plants produced by selfing show lethal characteristics and do not survive in high inbreeding depression.
• Inbreeding has a negative impact on fertility and even production.