Inbreeding:Its Meaning,Uses and Effects on Farm Animals

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various mating schemes of animals are classified under 2 broad categories,Inbreeding and Outbreeding.Classification depends on the closeness of the biological relationship between mates.Within each category a wide variation in intensity of this relationship exists.A very fine line separates the 2 categories.Mating closely related animals,for example:parent and offspring,full brother and sister or half brother and sister is inbreeding.With less closely related animals first cousin and 2nd cousins,people will always agree to disagree about where to draw the line between inbreeding and outbreeding.Technically,inbreeding is defined as the mating of animals more closely related than the average relationship within the breed or population concerned.Matings between animals less closely related than this,then would constitude as outbreeding.These 2 systems of mating,with varying intensities in each.
By:Dept. of Animal Sciences

[mikey]

Table 1:
degrees of inbreeding and outbreeding arranged according to biological relationship between indicated mates.In reading from top to bottom,biological relationship between mates steadily decrease.

Inbreeding-50% coeffient of relationship between mates-biological relationship between mates-parent x offspring,full siblings.

Inbreeding-25% coeffient of relationship between mates-biological relationship between mates-half sibs,double first cousins,aunt x nephew,uncle x niece.

Inbreeding-12 1/2% coefficent of relationship between mates-biological relationship between mates,first cousins.

Inbreeding-6 1/4% coefficent of relationship between mates-second cousins.

Inbreeding-?% coefficent of relationship between mates-biological relationship between mates-linebreeding-1.

Inbreeding/outbreeding- 0% coefficent of relationship between mates-biological relationship between mates-random mating within breed,population -2.

Outbreeding-0% coefficent of relationship between mates-biological relationship between mates,outcrossing

Outbreeding-0% coefficent of relationship between mates-buiological relationship between mates,breedcrossing.

Outbreeding-0% coefficent relationship between mates-biological relationship between mates,species crossing.

Outbreeding-0% coefficent relationship between mates-biological relationship between mates,genus crossing.

1)-In a linebreeding program,the coefficent of relationship between mates is usually low,but variable.
2)-Random mating within a breed or population means mates are chosen by chance.Inbreeding or Outbreeding could occur.                   
                   

[mikey]

Biological relationships between animals:
individuals are considered to be biological related when they have one or more common ancestor.For practial purposes,if 2 individuals have no common ancestor within the last 5-6 generations,they are considered unrelated.
Biological relationship is important in animal breeding because the closer the relationship,the higher the percentage of like genes,the 2 individuals carry.Closeness of relationship id determined by 3 factors.
1)-how far back in the 2 animals pedigrees the common ancestor appears.
2)-how many common ancestors they have.
3)-how many common ancestors appear.Also influnced by any inbreeding of the common ancestor or ancestors.
By: University of Missouri U.S.A.

[mikey]

Measurement of degree of biological relationship:
the coefficent of relationship is a simple numercial value that considers all the above mentioned factors.It is a measure of the degree to which the genotypes/gentic constitutions of the 2 animals

RBC=sigma 1/2 n+n' (1+FA) divide square root of 1+FB 1+FC
RBC=coefficent of relationship brtween animals B&C which we want to measure.(sigma=greek symbol meaning to add).
1/2=the fraction of an individuals genetic material that is transmitted to its progeny.Used in calculation of the coefficent of the relationship because it represents the probability that,in any given 1 generation,an identical gene from a given pair of genes is transmitted to each of 2 particular progeny.It is also the probability that an unlike gene from a given pair of genes is transmitted to the progency.
n=number of generations between animal B and the common ancestor.
n'=number of generations between animal C and the common ancestor.
FA,FB,FC,=inbreeding coefficents of the common ancestor and the animals B&C, respectively.
If none of the animals is inbred,the coefficent of the relationship is estimated as follows:
RBC=sigma (1/2 n divide n')
By: University od Missouri U.S.A.

[mikey]

Usefulness of coefficient of relationship information:
a livestock producer would find coefficient of relationship valuable in a number of ways.One may,for example want to sell an animal related to one that had sold for a high price.The higher the coefficent relationship between the 2,the better the sales point.Or one,may want to buy 1 or 2 related sires and one may cost more than one wishes to pay.If the coefficient of relationship between the 2 sires is high than one may be better off buying the lower priced sire over the higher priced sire.A practical use of the coefficient of relationship is estimating the performance value of untested animals.To estimate the value,one must know the performance value  of a related animal,the coefficient of relationship between the tested and untested animals and the average daily gain ADG of 2.25 lbs. or 1kg.Assume further that a sire from this herd had a ADG of 3.50 lbs. per day feedlot,while a younger 1/2 brother has not,as yet,been evaluated for feedlot ADG.Assuming no inbreeding the coefficent of relationship between the 1/2 brothers is 0.25.Best estimate of untested animals feedlot ADG is that it will deviate from the herd average 25% as far as does the performance value of the tested 1/2 brother.Using these figures,the most probable feedlot ADG value of the untested animal is:
2.25 + 0.25 x 3.50-2.25=2.56 lbs. or a little over 1kg. per day.

[mikey]

Genetic consequences of inbreeding:
the basic genetic consequence of inbreeding is to promote what is technically known as Homozygosity.This means there is an increase in the frequency of pairing of similar genes.Accompanying this increase there must be a decrease in the frequency of pairing of dissimilar genes.This is called a decrease in Heterozygosity.These simultaneous events are the underlying reasons for the general effects on performance we observe with inbreeding.
Reasons for inbreeding:
developement of highly productive inbred lines of domestic livestock is possible.Occasional high performance animals are produced.Generally believed the effects of inbreeding are poorer reproductive efficiency including higher mortality rates,lower growth rates,higher frequency of hereditary abnormalities.The extent of this decrease in performance in general is in proportion to the degree of inbreeding.The greater the degree of inbreeding the greater the reduction in performance.Some characteristics like meat quality are hardly influenced by inbreeding,others like reproduction effeciency are greatly influenced by inbreeding.Most inbreeding studies suggest,estimates of average increases in percentage inbreeding within a closed herd can be made.
By: University of Missouri U.S.A.

[mikey]

Summary:
the primary genetic consequence of inbreeding is to increase the frequency of pairing similar genes.All genetic and phenotypic changes associated with the practice of inbreeding stem,from this one primary consquence.Despite these generally harmful effects,inbreeding is a very useful tool in the field of animal breeding.It helps the breeder to uncover and eliminate harmful recessive genes within the population.Seed stock and commercial producers have successfully used linebreeding to maintain a degree of genetic relationship in their animals to some outstanding ancestor or ancestors.

[mikey]

These terms describe various mating schemes and the progeny resulting from specific mating programs:
Backcross-progeny resulting friom the mating of a 2 breed cross animal to one of the parental breeds.For example using 2 breeds designated as P1,P2, backcross progeny would be produced by mating the 2 breed cross animal P1xP2 with either of the P1,P2 parental breeds.
Crisscrossing-continous program of crossbreeding in which there is an alternate use of males belonging to 2 breeds.Using 2 breeds designated as P1 and P2 a crisscrossing program beginning with the 2 breed cross P1xP2 would begin by backcrossing to 1 of the parental breeds P1xP2xP3.Females resulting from these matings would be bred to a P2 male,P1xP2xP1xP2 and so on.
Crossbred-progeny resulting from the mating of outcross animals belonging to different breeds.
Genus cross-mating of animals belonging to different genera,example mating domestic cattle to American buffalo.
Grading-mating purebred males of a given breed to non purebred females and the resultant female offspring in successive generations.
Inbred line-line of animals produced by mating related animals.
Incross-progeny resulting from the mating of animals from different inbred lines within a breed.
Incrossbred-progeny resulting from the mating of animals from in -bred -lines of different breeds.
Outbreeding-mating of animals less closely related to each other than the average relationship within the breed or population concerned.
Outcross-progeny resulting from the mating of unrelated animals within a breed.
Species cross-mating of animals belonging to different species for example mating European cattle to Brahman cattle.
3-Breed Rotational Cross-a continous program of crossbreeding in which males of 3 different breeds are used on a rotational basis.Using 3 breeds designated as P1,P2 and P3,the first generation would involue production of 2 breed cross animals,P1xP2.In the 2nd generation,2 breed cross females would be mated to males of the 3rd breed.P1xP2xP3 breed cross females would be mated to males of one of the breeds used to produce the 2 breed cross animals.P1xP2xP3xP1 and so on.
Topcross-progeny resulting from the mating of animals belonging to different families within a breed.
Topcross bred-progeny resulting from the mating of inbred males to non inbred females of another breed.
Topincross-progeny resulting from the mating of inbred males to non in bred females of the same breed.
2-Breed cross-progeny resulting from the mating of males of 1 breed to females of another breed.
By;University of Missouri U.S.A.