# inbreeding?



## PPVallhunds (Jul 26, 2010)

just wondering peoples opyions on inbreeding mice, would u consider motherXson, fartherXdaughter, brotherXsister breedings ok? If you would do these breedings would you then outcross to an unrelated mouse, do another close mating or breed to a more distant relative? Or do you not agree with these sort of matings, or prehaps only for certain reasons?

Just curious realy as i know opyions on this can varie on species, people i know who breed dogs wont do the above mentioned matting but with guppies i was told to get show quality you have to inbreed and it doesnt seem to be a big deal at all.


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## SarahC (Oct 3, 2008)

I would do all of the pairings you list and rarely out cross.


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## WillowDragon (Jan 7, 2009)

I inbreed most of the time and rarely outcross.


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## SarahC (Oct 3, 2008)

Except for my hairless I must add who are not suitable candidates for to much inbreeding and have to be regularly outcrossed to prevent eye abnormalities.


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## geordiesmice (Oct 26, 2010)

I do mostly inbreeding same here with the hairless sarahc.


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## Rhasputin (Feb 21, 2010)

Inbreeding only causes problems, if there are problems there to begin with.


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## Jack Garcia (Oct 9, 2009)

You must inbreed if you want consistency. If you outcross (breed unrelated individuals) all the time, your mice will very very inconsistence in appearance but also in disposition, health, and other matters.

Most (or all?) varieties of show animals are highly inbred for this reason.


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## Roland (Aug 17, 2009)

I think there is no simple answer to this question. Therefore some time ago I have written a text for my homepage:
What is Inbreeding and what is it usefull for?
What is Inbreeding Depression ?
How could the laboratory inbred mouse strains develop nevertheless?
What is hybrid vigour (Heterosis effect) ?
What is Outcrossing ?
What is Line breeding ?
What has to be done if you want to use line breeding for your mousery ?

The full text is available as a pdf download here:
http://www.repage7.de/member/drofi/archives2010.html and scroll down to 2010-10-22

Here is a copy:

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.


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## PPVallhunds (Jul 26, 2010)

thats an very usefull atrical Roland.

Thanls for your honist opyion everyone.


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