Systems of animal breeding has been broadly divided under inbreeding and outbreeding. Animal breeding is a branch of animal science that addresses the evaluation of the genetic value. Animal breeding is used to controlled for propagation of domestic to animals in order to improve desirable qualities.
Systems of Breeding:
The ultimate aim of the breeder is to evolve outstanding and improved type of animals which can render better service to man. Selection and system of breeding constitute the only tools available to the breeder for improvement of animals. Since new genes cannot be created, though they can be recombined into more desirable groupings.
1. Inbreeding:
Inbreeding is a mating system in which individuals mated are more closely related than the average of the population from which they come. It means the mating of males and females which are related. Animals deemed to be related only when they have one or more ancestors in common on the first 4-6 generations of their pedigree.
The intensity of inbreeding depends upon the degree of relationship. Close inbreeding denotes mating of closely related individuals like dam to son (mother x son) or sire to daughter (father x daughter) or full brothers to full sisters.
Inbreeding makes more pairs of genes in the population homozygous. Wherever there is inbreeding, there will be one or more common ancestors from which, part of the gene samples (gametes) have arisen.
Inbreeding can again be divided into following groups:
(a) Close Breeding:
This means the mating of full sister to full brother or sire to his daughter or dam to her son. These types of mating should be only when both parents are outstanding individuals, and then only at the increased risk of bringing undesirable recessive genes into homozygous form in the progeny.
Advantages:
These are as follows:
i. Undesirable recessive genes may be discovered and eliminated by further testing in this line.
ii. The progeny are more uniform than out-bred progeny.
Disadvantages:
These are as follows:
i. The undesirable characteristics are intensified in the progeny if unfavourable gene segregation occurs.
ii. It has been observed that the progeny become more susceptible to diseases.
iii. Breeding problems and reproductive failure generally increase.
iv. It is difficult to find out the stage of breeding at which it should be discontinued in order to avoid the bad effects of the system.
(b) Line Breeding:
This means the mating of animals of wider degrees of relationship than those selected for close breeding. It promotes uniformity in the character. Homozygosity is not reached so quickly as in close breeding. Neither desirable nor harmful characters are developed so quickly.
It is a slowed method for the fixation of hereditary outstanding bull or cow and the progeny is mentioned as being line-bred to certain ancestors.
Advantages:
These are as follows:
i. Increased uniformity
ii. The dangers involved in close breeding can be reduced.
Disadvantages:
The chief danger in live breeding is that the breeder will select the animal for pedigree giving no consideration to real individual merit. This may in some cases result in a few generations which receive no benefits from selection.
Consequences of Inbreeding:
The effects of both close breeding and line breeding are similar. The only difference is in the degree of their intensity. They are more intense in close breeding and less so inline breeding.
These may be described as follows:
(I) It increases homozygosity (like alleles) and decreases heterozygosity (dissimilar genes) and hence favours the development of genetic uniformity amongst the animals.
(II) It is the best method of getting true strains from unknown stocks as it sorts out the characters in the homozygous condition and thus help in the selection of undesirable individuals and culling of the undesirable individuals.
Strains which breed true are not obtained in animals. However, and in plants are obtained only when close inbreeding is accompanied by intense selection.
(III) The outward effects of inbreeding may include the following:
(a) Effect on Growth Rate:
With the exception of a few laboratory animals, some moderate decrease in growth rate and mature weight has been noted. The studies on rats on the other hand have shown that they withstand inbreeding (up to 100 generations over 40 years) without any apparent bad effect on growth rate. The reproductive rate of rats is very high, however, relative to farm stock.
(b) Effect on Reproductive Performance:
In almost all the cases, a reduction in the reproductive efficiency has been noted. It may delay testicular development and puberty, reduce gametogenesis or increase the embryonic death rate.
(c) Effect on Vigour:
It has been noted that the death rates among the inbreeding groups of animals are higher than those in outbreeding ones. They are also adversely affected by environmental conditions. They have higher percentage of Runs (animals which are undesirable due to various reasons).
(d) Effect on Production:
Production traits usually show moderate decrease with increased inbreeding.
(e) Appearance of Abnormalities:
Hereditary abnormalities or lethal factors are likely to appear more often in the inbreeding animals than in the outbreeding ones.
2. Outbreeding:
It is the breeding of unrelated animals.
This involves the following types of breeding:
The genetic effects of outbreeding are opposite to those of inbreeding. Whereas inbreeding increases homozygosity, outbreeding tends to make more pairs of genes heterozygous.
Therefore, outbreeding animals are less likely to breed true than are inbreeding animals.
(i) Outcrossing:
Mating of unselected animals within the same pure breed is termed ‘Outcrossing’. The animals mated have no common ancestors on either side of their pedigree up to 4-6 generations and the off spring of such a mating is known as the outcross. Traits which are largely additively genetic and which have a high heritability, respond quickly to out crossing.
In a period of many years, the various breeds in many western countries registered progress mainly though a system of outcrossing in which the best available, but unrelated sires were continuously selected for use on the females in a herd or flock.
Advantages:
These are as follows:
i. This methods is highly effective for characters that are largely under the control of genes with additive effects i.e., milk production, growth rate in beef cattle, etc.
ii. It is an effective system for genetic improvement if carefully combined with selection.
iii. It is the best method for most herds.
(ii) Cross Breeding:
It is the mating of the animals of different breeds. It is generally used where the crossed progeny is directly marketed and are not needed for breeding and further multiplications. It has become quite common in pigs and in the production of hybrid chickens. With the beef cattle also it is practiced to a certain extent. Cross breeding for milk product has been tried with varying degree of success. It is generally used for production of new breeds.
The method of cross breeding is described below:
(a) Criss Crossing:
When two breeds are crossed alternatively, the method is known as criss crossing. This method is proposed for utilising heterosis in both dams and progeny.
Breed A females are crossed with breed B sires. The cross breed females are mated back to sires of breed A and so on. In this system, the cross breeds soon come to have about two-third of their inheritance from the breed of their immediate sire with one – third from the breed being used.
(b) Triple Crossing:
In this system three breeds are crossed in a rotational manner. It is also known as rotational crossing.
Three breeds are used in this system. The females of crosses are used on a sire of pure breeds in rotation. The cross breeds will soon come to have 4/7 of inheritance of the breed of the immediate sire, 2/7 from the breed of maternal grand sire and 1/7 of the hereditary material of the other pure breed.
(c) Back Crossing:
Back crossing is mating of a cross breed animal back to one of the pure parent races which were used to produce it. It is commonly used in genetic studies, but not widely used by breeders. When one of the parent possesses all or most of the received traits, the back cross permits a surer analysis of the genetic situation than F2 does.
A heterozygous individual of the F1 when crossed with a number of homozygous recessive parent race, the off spring group themselves into a phenotypic ratio of 1 : 1; if, on the other hand, the individual of the parent race were to be homozygous dominant all the off springs will be alike.
All being tall, there is no ratio of tall to dwarf.
Advantages of Cross Breeding:
These are as follows:
(i) It is valuable as a means of desirable characters into a breed in which they have not existed formerly.
(ii) It serves a good purpose in evolving a new breed owing to the fact that it disturbs the balance and brings about recombination in the germplasm to cause variations.
(iii) Selection can then fix the favourable variations in the population.
(iv) It is an extremely handy tool to study the behaviour of characteristics in hereditary transmission.
(v) The cross breed animals usually exhibit an accelerated growth and vigour or heterosis, which means the blending of desirable dominant genes from two breeds in the first generation. Such animals are thriftier than either of the parents.
(vi) They grow rapidly, produce more milk, etc. than would be expected from their pedigree. The productive traits usually show the greatest improvement from cross breeding.
Disadvantages:
These are as follows:
(a) The breeding merit of cross breed animals may be slightly reduced because of the heterozygous nature of their genetic composition, and the fact that all animals transmit only a sample half of their own genetic materials to their off springs. This may otherwise be explained that crossing has a tendency to break up established characters and destroy combinations of characters which have long existed in the strains and behaved in a manner like unit characters in transmission.
(b) Cross breeding requires maintenance of two or more pure breeds in order to produce the cross breed which undoubtedly involved a considerable investment as rapid progress need not be expected in any line of breeding unless sufficient numbers can be kept to allow for rigid selection.
(iii) Species Hybridisation:
Crossing of individuals from two species is called hybridisation. This is the widest possible out breeding system. It is possible only between related species which might have descended from common parent stock somewhere back in the evolutionary process. In the course of time, evolutionary forces might have set them off into distinct species.
A well-known example for species hybridisation is the mule which is a cross between the jack ass and the mare. The mule is valued for its ability to work hard under the most adverse conditions. The mules are generally sterile. Similarly, European cattle and American bison when crossed produce sterile males and fertile females. By back crossing the females to bison and cattle attempts are being made to form a new breed of cattle-the cattalo.
Species crosses are possible only when two species are related closely enough. These crosses usually result in much heterosis (a phenomenon in which the crosses of unrelated individuals often result in progeny with increased vigour (much above their parents). However, the progeny of species crosses have very low or nil fertility.
(iv) Grading Up:
It is the practice of breeding sires of a given breed to non-descript females and their offspring for generation after generation. Thus, grading up is a successive use of pure breeds bulls of a certain breed of a non-pure breeds herds. The continued use of good pure breed sires for only a few generations are all that are required to bring the herd to the point at which it has all the appearance, actions and practical value of pure breeds.
The following presentation table shows the rapidity with which pure breeds sires will change the genetic composition of the non-descript one.
From the above explanation, we can see that the off springs come closer to a 100% improved breed, as we go on breeding.
Grading is a process by which a few pure breeds sires can rather quickly transform a non-descriptive population into a group of pure breeds. The grading process does not create anything new but it may transfer the good qualities of an improved breed.
For a grading up programme, it is advisable to use a breed that has thrived well under local condition. Otherwise graded animals may not adapt themselves to the local environment.
Advantages:
These are as follows:
1. Pure breeds can be obtained just after a few generations (after 7th to 8th generations).
2. The start can be made with a little money in comparison to die purchase of an entire herd of pure breeds.
3. It helps to prove the potentialities of the sire and adds to its market value.
4. It is a good start for new breeders who can slowly change over to pure breed systems.
Disadvantages of Grading Up:
These are as follows:
1. Pure breeds are not always better than grade or country animals for the use to be made of them.
2. Pure breed stocks which give good results in one set of environmental conditions do not always give favourable results in some different environmental set-up. The pure breed dairy cattle from temperate zones often degenerate when used in tropical areas. Moreover, their off springs fail to show vigour and Constitution for high reproduction. To make grading successful the pure breed must have the ability to perform under the environmental set-up where their offspring is going to perform.