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Outbreeding

Genetic disassortative mating

Out breeding is the mating of animals which are less closely related to each other than the average of the population

Its general effects are the opposite of those of inbreeding. Out breeding increases the heterozygosity of the individual.

Out breeding systems are broadly classified as follows:

Out crossing
Top crossing
Line crossing
Grading
Crossbreeding
Species hybridization

OUT CROSSING

Out crossing usually applies only to mating within a pure breed. If two lines or flocks within the same breed are separated for four or five generations and the sire from one herd is used in another herd that amounts to out crossing.

The use of out crossing in purebreds are;

When there is lack of selection response due to reduced genetic variability.
To reduce inbreeding in a closed population.
To introduce new genes with reference - color, horn type, etc.

TOP CROSSING

Top cross usually refers to the best sire in a pedigree. Top crossing also refers to the continued use of sires to different families within a pure bred, same breed or different breed.

normally used within pure breeds. Top crossing refers to the use of highly inbred males to the females of the base population or non-inbred population

LINE CROSSING

It refers to crossing of inbred lines within a specific breed. Line crossing takes advantage of both increased homozygosity within a line and the difference between lines.

Line crossing is mainly done to exploit heterosis or hybrid vigour

Back crossing

mating of a cross bred animal back to one of the pure breed parent, which were used to produce it, commonly used in genetic studies, but not widely used by breeders. When one of the parents possess all or most of the recessive traits, the back cross permits a surer analysis of the genetic situation than the F₂ does.

Test cross: phenotypic ratio of 1:1.

GRADING / GRADINGUP

Grading up is the continual use of sires of one pure breed starting with foundation females of another breed or no particular breed at all (Non-descript). Marked improvement in crosses if sires from a particular breed (A) are repeatedly back crossed to another breed / non-descript animals (B). Five generations are sufficient to raise the level of inheritance of breed A to 96.9% (0.969) in the fifth generation. After five generations of repeated back crossing to a particular breed, the animals after the end of fifth generation become eligible to be registered as purebred.

Cross Breeding

mating of two individuals from different breeds. Cross breeding is done to exploit hybrid vigor or heterosis and to sell the crossbred to market. Every time, the parental breeds have to be crossed for producing market animal.

Crossbreeding has been used in recent years to establish a broad genetic base in the development of new breeds or synthetics: one or two crosses between the two or more populations are made in order to produce a single population of animals containing genes from each of the population involved. Once a synthetic has been formed then the main aim is to improve it as rapidly as possible by selection within it. For example: Santa Gertrudis, The Jamaica Hope, the Norwegian Red and White, the Australian Milking Zebu, Hissardale, Karan Swiss, Sunandhini, Taylor breed.

The main guidelines to be followed in crossing to produce a synthetic are:

Ensure that the animals used in the original crossings have been intensely selected in terms of relevant characters;

it is of no use starting a synthetic with inferior animals.

Maximize variance in breeding values amongst the foundation animals in the synthetics using as many unrelated animals as possible from each of the contributing populations.

Heterosis

Crosses of animals from different strains or lines of the same breed, from different breeds or from different species, result in offspring whose level of production is above that of the average of the parents.

The increased production may be due to increased fertility, increased pre and post natal viability, faster and more efficient growth, improved mothering ability etc. The increased level of performance as compared to the average of the parents is known as heterosis or hybrid vigour.

Heterosis is the phenomenon in which progeny of crosses between inbred lines or purebred populations exceed the average of the two parental populations.

It is just the opposite of inbreeding depression.

Heterosis = [(Mean of F1 off spring - Mean of Parents) / Mean of Parents ] x 100

Genetic basis of heterosis

Dominance Theory
Overdominance Theory
Epistasis Theory

SPECIES HYBRIDISATION

Most extreme form of out breeding

Hybrids can occur where the species are closely related for the egg and sperm to result in a viable embryo

Where the two species are very closely related, the hybrids may even been partially or fully fertile

By crossing two different species, sometimes we get good individuals. The mule is a good example of a commercially important species hybrid

GCA

SCA is the superiority of a particular cross over the average GCA of the two lines and it is due to non-additive genetic variance. GCA & SCA are expressed as variance&not as values

In above crosses, any particular cross then has an expected value which is the sum of the general combining abilities of its two parental lines.

The cross however, may deviate from this expected value to a greater of lesser extent. This deviation is called SCA of the two lines in combination.

Hybrids

Recurrent Selection

Recurrent selection was proposed by Jerkins (1940) for improving GCA in plant
It was further modified by Hull (1945) to utilize the specific combining ability

Definition:

Recurrent selection is defined as reselection generation after generation, with inter-mating of selected animals/ birds to produce the population for the next cycle of selection.

Here a highly inbred line is selected as tester “Tester line”
Large number of individuals of a source population are tested in crosses with this tester line, known to have good GCA.
There will be selection of male and female of the source population on the basis of their cross progeny of tester line performance.
Cross bred progeny are not used further for breeding
The selected individuals from source population are mated again and again among themselves to produce next generation of complementary stock of the tester inbred line.
This cycle is repeated several generations till maximum progress is achieved
By this method the GCA of only one line and SCA of cross are improved
Practical difficulty is to produce and maintain highly inbred tester line.

Reciprocal Recurrent Selection

Recurrent selection was proposed by Comstock, Robinson and Harvey (1949)

Later, Bell, mooren and and Warren (1950, 51) extended it to poultry for improving both GCA and SCA.

Definition:

A form of Recurrent selection used to improve both GCA and SCA of population for a character using two heterozygous tester line, is known as Reciprocal recurrent selection It is also known as recurrent reciprocal half sib selection.

Properties

Used for improvement in polygenic traits
Selection is made on the basis of test cross performance
Two heterozygous testers are used as source population
It is used for improving population of GCA and SCA for specific characters.
It is equally effective with all kind of intra-allelic interactions
It is used for improvement of character governed by AGA and NAGA.

Procedure:

It involves two segregating population.
Each will act as a source material for selection based on their crossbred performances
This cross helps to select for specific sequence of genes for higher performance thus helps to improve additive gene effect
This will also helps to retain partially the hybrid vigor which is due to non-additive gene action.

Recurrent Vs Reciprocal Recurrent Selection

RS is superior at beginning - more differences in gene frequencies of inbred tester and source population, than between the two segregating population used in RRS
RS is inferior, if the loci with additive or semi additive gene action prevailed
Both these systems are inferior to pure line selection for traits governed by additive gene effects with high heritability

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