Mating Systems

Classification of mating system

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 :

1. Random mating,
2. Genetic assortative mating,
3. Genetic disassortative mating,
4. Phenotypic assortative mating, and
5. Phenotypic disassortative mating.

Mating system is classified phenotypically on the basis of correlation between the mates.

Random Mating

• Simplest of all mating systems
• According to Lush (1937) mating is random “when mates are no more and no less alike, either as individuals or in pedigree, than if they had been mated by drawing lots from within the group selected to be parents”.
• This implies that any one individual of one sex is equally likely to mate with any individual of the opposite sex.
• Random mating further assumes equal fertility and viability of all the individuals.
• Random mating is ‘Mating strategy without any strategy’

Phenotypic assortative mating

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.

• PAM tends to create more genetic, phenotypic variation than random mating population, in the population undergoing the PAM tends to spread the distribution away from the centre towards the extreme. So, the phenotypic variation caused by the PAM normally considered as draw back.
• However, greater the genetic variation faster the genetic change. Eg. To increase dairy milk yield, mating the high producing cows to bulls with highest predicted performance.

Negative assortative mating (NAM): Mating between dissimilar individuals. eg. Breeding best to worst.

• NAM or disassortative mating is mating like with unlike.
• NAM tends to decrease the variation i.e. intermediate types are produced due to mating of such individuals. It is not good strategy if we want to speed up directional genetic change. It reduces genetic variation, decrease response to selection. eg. In layers Rooster having high breeding value for egg size mated with hen with small size eggs.

Properties of assortative mating

• Sewall Wright (1921) studied and formulated same properties
• With complete PAM complete homozygosity of population is obtained but slowly
• Phenotypic assortative mating may had a population of genetic composition may different from that reached by inbreeding based on genetic relationship.

Genetic Assortative Mating / Inbreeding

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

• Close Inbreeding: Mating between sibs or between parents and progeny in order to achieve inbred lines with relatively high degree of homogenisity. In most of the time we use FS mating method. The same effect can be achieved by consistently back crossing the progeny to the younger parents. HS mating is much slower, rich in homozygosity but it is also less risky.
• Line breeding: It is a system of mating in which the relationships of an individual or individuals are kept as close as possible to some ancestor. In general line breeding is a milder form of inbreeding. As a general rule sire is not mated to its daughters but HS matings are made among the offspring of the particular sire. Used extensively in the past in development of British breeds of cattle such as Angus, Hereford and Shorthorn.

Precaution while applying inbreeding

• Should be practiced in purebred population of high degree of excellence, after identifying outstanding individuals.
• Probably most useful when an out standing sire is dead or not available for breeding
• To form new breeds, line breeding can be advocated.
• Tends to make gene good or bad, homozygous rapidly. Hence choosing of a ancestor (sire) to line breed is very important.
• Those that are definitely superior should alone be selected. Beside rigid selection, culling of undesirable recessive is highly essential.
• Should be practised only in herds distinctly superior to the general average of the breed.

Genetic Effects of Inbreeding

• It increases the probability that offspring will inherit the same genes from their parents.
• It makes more pairs of genes in the population homozygous regardless of the kind of gene action involved. The speed with which homozygosity is reached in animals depends upon the degree of relationship between mated individuals.
• It does not change the frequency of genes in the population but causes an increase in the frequencies of homozygous genotypes and consequently a decrease in the proportion of heterozygous genotypes.
• Inbreeding results in the fixation of genes whether their effects are favourable or unfavourable.

Phenotypic Effects of Inbreeding

• When the animals are homozygous for a no. of traits, the regularity of inheritance is assured (i.e it fixes the characteristics)
• Inbreeding reduces vigour is called inbreeding depression.
• Increased inbreeding results in:
1. Reduced fertility,
2. Reduced mothering ability,
3. Reduced viability and growth rate
4. Inbreeding if accompanied by selection may increase the phenotypic uniformity

Prepotency

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.
• 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

• Inbreeding and increasing of homozygosity is the only means of mating animals prepotent for characteristics.
• The more the animals are inbred, more they become homozygous for a no. 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

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.

Development of line

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.

Disadvantages of Inbreeding

• Undesirable traits appear with increasing frequency as intensity of inbreeding increases (lethal and sub lethal).
• Growth rates in farm animals reduced by inbreeding.
• Inbreeding reduces the reproductive efficiency.
• Reduced vigour lower vitality due to inbreeding depression

Inbreeding is only practiced when,

• the herd is better than the average. I.e when the frequency of desirable genes are more
• the herd has an outstanding sire
• the breeder knows the merits and demerits of inbreeding
• the herd is not maintained for commercial purpose.

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

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).

Genetic disassortative mating / Out breeding

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:

1. Out crossing
2. Top crossing
3. Line crossing
4. Grading
5. Crossbreeding
6. 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.

LINE CROSSING

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 / GRADING UP

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