INBREEDING - LINE BREEDING - OUTCROSSING
What is Inbreeding and what is it usefull for?
Inbreeding is the reproduction from mating of two genetically related parents.
Breeding in domestic animals is selective breeding primarily. Without the sorting of individuals by trait, a breed could not be established, nor could poor genetic material be removed.
Inbreeding is used by breeders of fancy mice to fix desirable genetic traits within a population or to attempt to remove deleterious traits by allowing them to manifest phenotypically from the genotypes. Inbreeding is defined as the use of close relations for breeding such as mother to son, father to daughter, brother to sister.
Systematic inbreeding and maintenance of inbred strains is of great importance for biomedical research. The inbreeding guarantees a consistent and uniform animal model for experimental purposes. Therefore many inbred mouse strains have been constructed, an example is the strain C57bl. Per definition a strain can be called inbred strain, after more than 20 generations of brother x sister matings. Many of the wellknown inbred strains go back to 1930, so they are stable strains now after more than 200 generations of brother x sister matings.
What is Inbreeding Depression ?
Inbreeding may on one hand result in more recessive deleterious traits manifesting themselves, because the genomes of pair-mates are more similar: recessive traits can only occur in offspring if present in both parents' genomes, and the more genetically similar the parents are, the more often recessive traits appear in their offspring. Consequently, the more closely related the breeding pair is, the more homozygous deleterious genes the offspring may have, resulting in very unfit individuals. For alleles that confer an advantage in the heterozygous and/or homozygous-dominant state, the fitness of the homozygous-recessive state may even be zero (meaning sterile or unviable offspring).
As a result, early generation inbred individuals are more likely to show physical and health defects, including:
• Reduced fertility both in litter size and sperm viability
• Increased genetic disorders
• Lower birth rates
• Higher infant mortality
• Slower growth
• Loss of immune system function
Another mechanism responsible for inbreeding depression is overdominance of heterozygous alleles. This can lead to reduced fitness of a population with many homozygous genotypes, even if they are not deleterious. Here, even the dominant alleles result in reduced fitness if present homozygously.
How could the laboratory inbred mouse strains develop nevertheless ?
Inbreeding depression is not a phenomenon that will inevitably occur. Given enough time and a sufficiently (but not too) small gene pool and a high number of individuals, deleterious alleles may be eliminated by natural selection by and by.
In a population where inbreeding occurs frequently, most offspring will have some deleterious traits, so few will be more fit for survival than the others. In populations with a large numbers of matings, the fittest will be selected and will survive. Breeders must cull unfit breeding suppressed individuals and/or individuals who demonstrate either homozygosity or heterozygosity for genetic based diseases.
In laboratories the high number of animals allows the eradication of deleterious traits and selection of the fittest, while in smaller populations of private mouseries the inbreeding depression often results in a dead-end street, without a chance to move back.
What is hybrid vigour (Heterosis effect) ?
Heterosis, or hybrid vigor (or outbreeding enhancement), is the increased function of any biological quality in a hybrid offspring. It is the occurrence of a genetically superior offspring from mixing the genes of its parents.
Heterosis is the opposite of inbreeding depression, which occurs with increasing homozygosity. The term often causes controversy, particularly in terms of the selective breeding of mice, because it is sometimes believed that all crossbred mice are genetically superior to their parents; this is true only in certain circumstances: when a hybrid is seen to be superior to its parents, this is known as hybrid vigor. When the opposite happens, and a hybrid inherits traits from its parents that makes it unfit for survival, the result is referred to as outbreeding depression.
Two competing hypotheses, not necessarily mutually exclusive, have been developed to explain hybrid vigor. The dominance hypothesis attributes the superiority of hybrids to the suppression of undesirable (deleterious) recessive alleles from one parent by dominant alleles from the other. It attributes the poor performance of inbred strains to the loss of genetic diversity, with the strains becoming purely homozygous deleterious alleles at many loci. The overdominance hypothesis states that some combinations of alleles (which can be obtained by crossing two inbred strains) are especially advantaggeous when paired in a heterozygous individual. The concept of heterozygote advantage/overdominance is not restricted to hybrid lineages. This hypothesis is commonly invoked to explain the persistence of many alleles which are harmful in homozygotes; in normal circumstances such harmful alleles would be removed from a population through the process of natural selection. Like the dominance hypotheses, it attributes the poor performance of inbred strains to a high percentage of these harmful recessives.
What is Outcrossing ?
Outcrossing is the practice of introducing unrelated genetic material into a breeding line. It increases genetic diversity, thus reducing the probability of all individuals being subject to disease or reducing genetic abnormalities by inbreeding depression. but it actually can serve to increase the number of individuals who carry a disease recessively, so has to be done carefully.
It is used in line-breeding to restore vigor or size and fertility to a breeding line.
Outcrossing is now the norm of most purposeful breeding, contrary to what is commonly believed. The outcrossing breeder intends to remove unwanted traits by using "new blood" with better traits. With dominant traits, one can still see the expression of the traits and can remove unwanted traits whether one outcrosses, line breeds or inbreds. With recessives, outcrossing allows for the recessive traits to migrate across a population. It may actually increase the number of individuals carrying a disease. This technique of 'improving' fancy mice by outcrossing should therefore be persued with some caution and should never be applied to the entire stud until the full effects have been studied carefully. A great deal of selection has gone into the varieties now available and, of course, selection has been for different characteristics in different varieties. So whilst improving one aspect of the stock byan outcross may well bes setting other aspects back for many years.
However one may increase the variance of genes within the gene pool by outcrossing, protecting against extinction by a single stressor from the environment.
As we will see, a good mix of methods is linebreeding, which seems to be the best way to improve a bred and to combine the advantages of inbreeding and outcrossing: Line breeding
What is Linebreeding ?
Linebreeding is a form of inbreeding practiced by most successful breeders to "fix" desirable traits in a bred of animal, without as high a risk of producing undesirable traits that may occur with close inbreeding.
The Inbreeding Coefficient makes the difference between inbreeding and line breeding. The coefficient of kinship is defined as the probability that the alleles at a particular locus chosen at random from two individuals are identical. In the mouse, the coefficient of relationship between parent and offspring is 50%, meaning half of the offspring's genome comes from that parent. Half the genes are identical by descent. The inbreeding coefficient of pairing parents x offspring is 25%. The inbreeding coefficient is roughly half the relationship coefficient.
Line breeding differs from inbreeding by the pairings, which are allowed.
In linebreeding it is not allowed to breed for inbreeding coefficients higher than 12.5%. This method is even helpfull to establish recessive traits. The homocygous offspring showing the recessive trait has to be selected.
Not allowed for linebreeding:
• parent x offspring 25%
• full siblings x full siblings 25%
Allowed for linebreeding:
• grandparent X grandchild 12.5%
• half siblings x half siblings 12.5%
• first cousins x first cousins 6.25%
• great grandparent x great grandchild 6.25%
• second cousins x second cousins 3,125%
Linebreeding fixes desirable traits and allowes the introduction of superior traits. Therefor this is the best method to improve the quality of lines in a mousery.
What has to be done if you want to use line breeding for your mousery ?
Linebreeding in private mouseries consists of setting up two or three lines per variety from a common initial stock. Each line operates separately from the others and the objective is to try to select for improved features whilst as far as possible avoiding common parents. Once the lines are well established, since they all have been derived from a common initial family or a single buck, they can be used as outcrosses for one another - when it seems necessary - thereafter returning to the former system of line breeding and selection. If you like, line breeding is rather less severe than inbreeding. One mates members of the same line or strain rather than near relations of a family group.
Start withn two not clesly related very good bucks and several very good but not closely related does. From the offspring start backcrosses as described under line breeding above.
A very occasional cross outside the strain may be used to correct or improve particular features or weaknesses, but this should be done carefully and selectively. Wholesale outcrossing which newcomers sometimes resort to, ususally results in a considerable deterioration in the medium to long term.
The text is an excerpt of several Wikipedia articles mixed with own experiences
from Chilloutarea Mousery http://drofi.repage7.de/