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üOnce the animals are selected to be the parents of the next generation, the breeder must decide upon the manner in which these are to be mated.
üAs per Wright (1921) the five basic types of mating systems are :
Mating system is classified phenotypically on the basis of correlation between the mates.
Mating system based on phenotypic resemblance. Also known as assortative mating, where mates are chosen on the basis of external appearance in a particular character.
Positive assortative Mating (PAM): Mating of phenotypically similar individuals (i.e like with like mating). eg. Mating biggest with biggest ; Mating smallest with smallest.
Negative assortative mating (NAM): Mating between dissimilar individuals. eg. Breeding best to worst.
Properties of assortative mating
The individuals being mated are more closely related by ancestry than they would be if they were chosen at random from the population.
This system of mating is usually called inbreeding.
The individuals are considered to be closely related if they have common ancestor in the previous four to six generations.
Inbreeding has been used in the past, especially in the early development of some breeds of livestock.
The early history of the Sorthorn and Thorough breed shows that considerable to intense inbreeding was practiced.
Mating system based on Genetic Relationship
Inbreeding: Mating between animals, which are more closely, related each other than the average relationship between all individuals in a population or inbreeding is mating between animals related by ancestors.
Inbreeding is classified into two types
Precaution while applying inbreeding
Ability of the individual to stamp its characteristic on its offspring to such an extent that they resemble their parents more closely than in usual
Measure of Prepotency
A strain could be defined as a group of birds or animals which have been closed for outside breeding and the herd or flock has been randomly mated with intense selection for a particular trait or traits for 5 generations and given a name.
When the population of animals closed for outside breeding, the population becomes closed flock. Estimate the genetic parameters, once the average performance of the closed flock are known, rigid selection is followed to improve particular trait in subsequent generations. The selected strain should have superior breeding quality.
In breeding point of view are more or less isolated from each other. Since the populations are close from the entry of new animals, homozygosity increases as a result of small population size. The superior strain formed within the breed could cross among them for exploiting heterosis or hybrid vigour.
Line can be defined as a collection of animals, as a result of inbreeding or more closely related to each other than the individuals in the strain. The line should be always qualified by inbreeding coefficient.
From the strain, the birds are chosen at random. Full sib or half sib matings are taken for successive generations. The progeny has a co-efficient of inbreeding for excess of 50%. Then perform selection among the population and fix a particular trait in that line which is homozygous for a particular trait.
Inbreeding is only practiced when,
The most striking observed consequence of inbreeding is the inbreeding depression. It is the reduction in the mean phenotypic value shown by characters connected with reproductive capacity or physiological efficiency. In general inbreeding tends to reduce the fitness. Thus, characters that form an important component of fitness, such as litter size show reduction on inbreeding. Whereas characters that are not closely related with fitness show little or no change.
Inbreeding depression for a single locus can be expressed as follows:
MF = Mo - 2dpqF
Where,
Mo - Mean value of a population for a particular character before inbreeding.
MF - Mean value of the population for a particular character after inbreeding.
F - Inbreeding co.efficient
d - dominance, i.e heterozygote does not have a value average to that of homozygote
p - Frequency of one allele
q - Frequency of other allele
Therefore, inbreeding depression, depends on dominance (d), inbreeding coefficient (F) and relative frequencies of alleles (p & q).
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 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.
Top cross usually refers to the breeding of 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
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
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 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.
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.
The cross breeding can be practiced in different ways depending on number of breeds used and the manner of their crossing:
1. Regular or systematic crossing
This type of crossing means mating of same type on regular basis to take the advantage of heterosis and complementarity. The crossing exploits non-additive gene effects through heterosis and the sdditive gene effectsthrough complimentarity when two or mor charecters complement each other.
There are two basic methods of regular crossing:
A. Specific crossing: It is further of two types,
a) Two breed crosses: Two pure breeds crossed together. This can be further divided in two following types,
(i) Two pure breed crosses: Two pure breeds are crossed together and their crossbred progenies are not used for further breeding. This crossing will produce progeny which are purely heterozygous display 100% individual heterosis but no parental heterosis as parents are not heterozygosis.
(ii) Inter se mating: This is cross between F1 and F1. Progeny produced will show all three types of heterosis, individual, maternal and paternal.
(iii) Back Crossing: Here in this case mating of crossbred (f1) is done with purebred animal of either breed. This is generally done to exploit both maternal or paternal heterosis. If maternal heterosis is necessary, crossbred parent should be females that means F1 should be crossbred female. The back cross progeny are, on an average, 50% less heterozygous than F1 and hence they show only 50% of individual heterosis.
(iv) Criss crossing: It is similar to back crossing but here both the parents P1 and P2 are used alternatively.
b) Three breed cross: Here three breeds are used. The F1 (Product of AxB) is mated to third breed like female of F1 crossed to C males will results in (ABC). The resultant will utilise full matrernal heterosis because the dam is 100% heterozygous being crossbred as well as of individual heterosis because progeny produced will be 100% heterozygous.
c) Four breed crosses/ double two breed crosses: This involves the crossing of crossbred females produced by crossing of two breeds (A and B) with cross of males produced by crossing of C and D
B. Rotational crossing: The males of two or three breeds are used regularly (rotation) in successive generation on crossbred females of the previous generation. Thiis actually rotational crossing which may involve two or three breeds.
2. Composite Crossing
This is an alternate to regular crossing. In this cross it is done by producing one or few crosses between two or more populations to produce a single population having genes from each of population. This single population of a mixture of various crossbred population is called composite or synthetics.
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.
Chimeras are not the same as hybrids. Hybrids have intermediate features and each cell is a mix of chromosomes from the parental species. Chimeras are a mix of genetically different cells to form a mosaic animal.
By crossing two different species, sometimes we get good individuals. The mule is a good example of a commercially important species hybrid.