Breeding means, the manner in which selected males and females are mated. Breeding makes new combination or sequencing of genes in the individual.
The breeders identify and select desirable qualities in animals for future mating and discard less desirable qualities.
For the improvement of live stock (= farm animals) selection and breeding must be practised simultaneously.
Continuous selective breeding leads to homozygosity in a population resulting a loss of variability. If all the individuals are alike, the breeder cannot make progress in future. Hence, there is a need to create a variability in population. This can be achieved by breeding. Therefore, selection and breeding go hand in hand for the improvement of live stock.
Classification of Breeding Systems:
Under the selected breeding system selected males and females are mated. The breeding system can be classified into five different ways depending on their phenotypic and genotypic relations.
The five breeding systems are:
(1) Random mating,
(2) Phenotypic assortive mating,
(3) Phenotypic dissortive mating,
(4) Genetic assortive mating and
(5) Genetic dissortive mating.
1. Random mating or Panmixia:
It is a system of mating in which each male individual has an equal opportunity to mate with the female individual and vice versa. This mating system generally takes place in nature where the number of males and females are assumed to be equal.
2. Phenotypic assortive mating:
In this type of mating animals which are phenotypically alike are allowed to mate among themselves. This is also called “like to like” mating.
3. Phenotypic disassortive mating:
Here individuals which are phenotypically unlike are allowed to mate. It is also called “unlike to unlike” mating. For example, mating of tall with short individuals.
4. Genetic assortive mating:
In this system individuals, which are closely related genetically are allowed to mate. This is also known as inbreeding.
5. Genetic disassortive mating:
This system is just opposite to the previous system where mating takes place between less closely related individuals. This is also called as out breeding.
Breeding Methods:
There are two major breeding methods: inbreeding and out breeding.
Inbreeding:
It is defined as “breeding of more closely related individuals (males and females) than the average relationship of the population.” Depending upon the closeness among mated individuals, inbreeding are of 3 types.
They are:
(i) Close inbreeding (mating individuals have relationship above 0.25),
(ii) Mild inbreeding (mating of relatives beyond 2nd generation and upto 6th generation),
(iii) Line breeding (mating of relatives between 4th-6th generations).
Advantages and Disadvantages
Advantages of Inbreeding:
1. Due to increase in homozygosity, the stamping ability or prepotency of inbred line increases.
2. It helps to eliminate lethals and semi lethals due to homozygosity.
3. It increases genetic variance between lines and reduces genetic variance within lines.
Disadvantages of Inbreeding:
1. Many lines are lost due to homozygous lethals or semi lethals.
2. Due to loss of heterozygosity, the hybrid vigour is lost.
3. Inbreeding leads to lower birth weight, post natal mortality (baby death after birth), poor growth, reproductive disorder and low resistance to diseases.
Out-breeding:
It is opposite of inbreeding where unrelated individuals are mated. The breeding individuals have relationship less than the average relationship of the population. Outbreeding results in increase in heterozygosity and decrease in homozygosity.
Out-breeding can be classified into two major classes:
(i) Out-breeding within a breed and
(ii) Out-breeding between two species/strain/line/breed.
Advantages of out-breeding:
1. Out-breeding increases heterozygosity which results in hybrid vigour (increase in weight, faster growth, increased resistance to disease, low mortality).
2. It covers the defects of recessive lethals and semi-lethal genes.
3. It increases genetic variance within lines.