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Prepotency is the 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. It is the property of the characteristic and not the individual breed or sex. When two individuals are mated one may have more influence than the other on offspring. Similarly some lines and breeds are more pre-potent than others. However prepotency can’t be passed on from one generation to another unless it is possessed by both sires and dams.
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A high degree of homozygosity and possession of a high per cent of dominant genes are the inherent qualities that will enable an animal to stamp its own characteristic on majority of its offspring. A perfectly homozygous animals produce only one kind of gametes and all the offspring will receive exactly the same gene from each. Any genetic difference between the offspring would depend entirely on the halving process and on number of different genes received from the other parent. If the parent is homozygous for several dominant gene all the offspring will resemble it irrespective of what they received from other parent. Here prepotency is the maximum.
Measure of prepotency
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In breeding and increasing of homozygosity is the only means of mating animals prepotent for characteristics. The more the animals are inbred the more they become homozygous for a number of genes. The inbreeding coefficient then is the best estimation of animal’s prepotency. Prepotency however is not transmissible from parent to offspring.
Development of Strain
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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 give a name. The description of the strain should always followed by a economic trait. This is considerably milder form of inbreeding in which strain forms. 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.
Development of Line
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The 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.
Uses of Inbreeding
In spite of certain obvious disadvantages of inbreeding, there are certain instances where it may be used as advantage of livestock production.
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The most practical use of inbreeding is to develop strains and lines that can be used for crossing purposes to exploit heterosis.
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Inbreeding may be used to determine the actual genetic worth of an individual, is done by mating to a sire with 25 to 35 daughters before it is used extensively in AI programme.
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Inbreeding could be used as a practical way to select against the recessive genes of economic importance. Such inbreeding brings out the hidden recessive genes both recessive homozygous and heterozygous parents can be identified and culled.
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Inbreeding may be used to form distinct families with in a breed especially the selection is practiced along with it.
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To maintain genetic purity and thereby to increase prepotency
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To eliminate undesirable recessives. When a sire is mated to 20 of its daughters and does not produce any recessive characters in the offspring, it may concluded that the sire is not heterozygous for recessive characters.
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To develop inbred lines.
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To regroup the genetic material
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To produce uniform progeny
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To determine the type of gene action. If inbreeding effects are large, the type of gene action is non – additive: if inbreeding effects are small , then the type of action is additive.
Disadvantages of Inbreeding
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Undesirable traits appear with increasing frequency as intensity of inbreeding increases (lethal and sub lethal).
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Growth rates in farm animals reduced by inbreeding.
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Inbreeding depression
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 and for all loci concerned
it is, MF = Mo - 2 F pqd
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 is – 2F pqd which depends on dominance (d), inbreeding coefficient (F) and relative frequencies of alleles (p & q). Genes are at intermediate frequency at the beginning of breeding show highest depression. Economic traits like reproductive viability, milk yield and growth rate show inbreeding depression. Characters like fat % and back fat thickness do not show much inbreeding depression.
Inbreeding is to be practised only when
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the herd is better than the average. I.e when the frequency of desirable genes are more
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the herd has an outstanding sire
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the breeder knows the merits and demerits of inbreeding
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the herd is not maintained for commercial purpose.
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