An animal ‘breed’ refers to a specific group of animals coming from the same lineage or a related one, having mostly similar characteristics like homogenous behaviour, appearance, features, size, and so on. Animal breeding is an essential part of animal husbandry. It refers to controlled mating of individuals of the same breed or different breeds, followed by selection to obtain a superior genotype of domesticated animals. Breeding of animals is done to increase the yield of animals; better quality of animal products; a higher reproductive rate; and resistance to various diseases. 

Mating of animals can be done in two ways: natural propagation/mating or artificial insemination. To make progress in breeding successive generations, genetic variation is necessary. There are two systems of animal breeding: When breeding is between closely related individuals or different individuals of the same breed, it is referred to as inbreeding and when the breeding is between non-related breeds, it is called outbreeding. 

Overview of inbreeding 

Inbreeding involves the mating of closely related animals of the same species, within the same breed, for successive generations. Individuals having one or more common ancestors are considered to be biologically related. The inbreeding system involves the identification and selection of superior males and superior females, preferably from different populations, but from the same breed, who are then mated in pairs. The superior males and females from the progeny obtained from such mating are evaluated for desirable traits and are selected for further mating. Among cattle, a superior cow or buffalo is the one that gives more milk per lactation. A superior bull is referred to as the bull that gives birth to superior progeny in comparison to other males. The process is repeated for 4-6 generations. Bulls found unsuitable for breeding are castrated when they are young, so they become bullock which then acts as the main draught power in India.

In cattle, inbreeding often results in the magnification of a desirable trait at the cost of another. For example, the inbreeding of Holstein dairy cattle has led to an increase in milk production; however, the cows are more difficult to breed.

Selection of individuals for inbreeding

In inbreeding, genetically superior individuals having common ancestors are purposely mated so that there is an increased chance that the offspring obtained from such mating will receive the same allele from both parents. The parents share alleles because they have a common ancestor who has passed on the same alleles to multiple offspring, who in turn passed them on again so that eventually they ended up in both related animals. If an individual receives identical genes from each parent, he/she shows homozygosity for that pair of genes. If the gene received by the individual from each parent leads to superior production, then mating is desirable. 

Types of inbreeding

Inbreeding can be further categorised into: 

1. Close inbreeding: In this type of inbreeding, the mating of very close relatives, such as between parents and offspring (first-degree relatives) and between full brothers and sisters, is opted for. It also involves mating between second-degree relatives such as grandparents and grand-offspring, half brothers and sisters, uncles/aunts and nephews/nieces, and double-first cousins. 
2. Line inbreeding or linebreeding: It involves the mating of animals that are not so closely related but have at least one common ancestor in the pedigree. For example, cousin grandparents to grand-offspring. In linebreeding, there is a planned attempt at maintaining or increasing the relationship between the offspring and a specific desirable ancestor. Thus, linebreeding aims at maintaining a close connection between individuals in the pedigree while keeping inbreeding as low as possible. 

Advantages of inbreeding

• Inbreeding leads to increased homozygosity which signifies an increase in the frequency of pairing of similar genes and a consequent decrease in heterozygosity. Thus, to evolve a pure line in any animal, inbreeding is necessary.
• In inbreeding, the sperm or egg cells’ genotypes from inbred individuals are more predictable than that from outbred ones.

• Inbreeding leads to exposure and elimination of harmful recessive genes.
• Inbreeding helps in the accumulation of superior genes. Through selection, less desirable genes can be eliminated, resulting in an increase in the productivity of the inbred population. 

Disadvantages of inbreeding

• Inbreeding decreases resistance and milk production in dairy cows.
• The negative aspects of inbreeding involve poor reproductive efficiency, high mortality rates, low growth rates, and a high frequency of hereditary abnormalities.
• Inbreeding leads to a reduction in genetic diversity.
• Continued inbreeding, especially in closely related individuals, usually results in reduced fertility and productivity. 

Inbreeding depression 

The progeny obtained from inbreeding shows homozygosity for any identical gene received from both parents, so there is a high chance that the animal may express undesirable genes, and hence, undesirable traits. Inbreeding does not lead to the creation of undesirable recessive genes; however, it does expose these unfavourable genetics. This leads to a decline in average phenotypic performance such as reproduction rate and yield which is termed inbreeding depression.  To overcome this problem and help restore fertility and yield, it is recommended that selected animals of the breeding population should be mated with superior animals of another unrelated population, but of the same breed. 

Conclusion

Inbreeding involves the mating of same-breed individuals that are more closely related than the average of the breed. Inbreeding is of two types: close inbreeding and line inbreeding. Close inbreeding involves mating between first-degree relatives and even between second-degree relatives. Linebreeding involves the mating of animals that are not so closely related but have at least one common ancestor in the pedigree.

Inbreeding promotes homozygosity and consequently reduces heterozygosity. It also helps in the elimination of harmful recessive genes. However, inbred animals have poor reproduction efficiency, reduced milk production, and growth rate, which is referred to as inbreeding depression.