Inbreeding, Linebreeding & Outcrossing
Written by Yvette Mackail CVN.

The inbreeding issue is one of the most misunderstood practices among fanciers in many fields

  Sensible, selective breeding is a must for all breeders wishing to improve, or even to maintain lines of healthy, happy rats & mice.  The term "line breeding"(aka "inbreeding") has a stigma attached to it, where many people will dis-own you if they think you're breeding in this way.  However they are under false opinion of this practice.   

In animals such as race horses, some breeds of dogs, cats and even cavies, and many other animals, even humans,  problems arise in almost every instance of inbreeding.   Many of you will have heard people say "inbreeding causes problems"

FACT - The practice of inbreeding does not 'create' the resulting condition.  What it does do, is identify any bad genes if they are present in the line.   The problems must already be present in the line for bad results to occur.  

The sheer fact that so many breeders have shied away from inbreeding is part of the initial problem

When inbreeding is not practiced, any lethal, semi-lethal & undesirable recessives present in that line, are being passed down to future generations along with the good genes, without any apparent signs to you, the breeder.  With the end result being, that a majority of animals of a particular line/breed/species will all be carrying these undesirable genes.  

  The insight that inbreeding gives into your lines, is invaluable.  If you don't know about any bad traits, should they be present, then how can you expect to eliminate them.   It's the responsibility of all serious breeders to ensure that their lines of animals are genetically sound.  Working towards improving all the aspects of a breed is what the fancy is all about.   Bad genes not only affect the aetiological and health areas of your chosen breed, but temperament as well.  

There are many breeds of animal nowadays that are rarely if ever inbred because of the undesirable outcome.  With rodents being a fairly new fancy here in Australia, we need to pay attention to starting things off the right way from the beginning.   By inbreeding and thus identifying any bad recessives in our lines, we can weed them out, and breed healthier, more robust strains of rats and mice.
  This is not to say that our lines are without problems here.  There have been a few problems diagnosed already in some of them.  Both rats & mice.  But rather than hide these problems and not mention them to anyone, we need to  fix them if we can.  


It's an age-old story, the one about the enthusiastic new fancier who's just joined your club and is showing so much promise........
Then one day you hear about this poor unfortunate soul, being ridiculed by the so-called responsible breeders in the group, because he decided, against the advice of his fellow fanciers, to inbreed some animals to improve type, colour, pattern or maybe even temperament.  
  Yes he gets the improvement of the colour etc., that he set out to do, but all of a sudden he has these sick or diseased or deformed animals in the litters.  Maybe even stillbirths.   Not knowing where they've come from or how this could have happened.  
  He's being ridiculed for doing the "wrong thing" by inbreeding, and apparently "causing" these horrid conditions in the babies, when the blame really should fall on whoever it was that developed that line in the first place.  What this new breeder has actually done without even knowing it, is perform a great service to the fancy, by illuminating these problems, so that now, work can begin towards eradicating them if possible.  Other fanciers with animals from the affected lines are now aware of the possible problems also.  


Outcrossing can be a very valuable practice and there are times when it is essential, like for example manx rats.  The problems that can occur in this breed come about because of the gene effecting the lack of tail.  By breeding related offspring together you compound the genes effects, and the result can be disastrous.  So while these problems won't be technically bred out, they will be managed, and the resulting animals will be healthy.  Outcrossing to completely unrelated animals is a must with this breed, due to the severity of the problems that can occur.  It's rarely if ever suggested to linebreed or inbreed with manx rats.
To aid the cause, ANRA (the Australian National Rodent Assoc.) and most other rat organisations that recognise manx as a breed, have it in their standard that the manx can be shown in any standardised or unstandardised pattern and/or colour.   Taking away the most common reason for inbreeding, that being for improvement of colour and/or markings.  

   But it needs to be noted that there will be times when the bad traits identified will prove impossible to breed out or manage, simply because the sheer nature of the problems are grave.   Times where the genes responsible for a particular breed/marking etc. will be inseparable from the undesirable results they cause.   In severe instances such as this, the discontinuance in breeding of this line is essential.   Outcrossing in cases like this usually does not help the line either.  Because the good and the bad are inseparable, outcrossing simply produces animals without the desired phenotype and  guarantees the recessives will be carried on, and then we're right back where we started.

  I feel I should back up the above paragraph with case evidence (referred to as the pcd line).   About a year ago, a line of mice here were noted to be dying off at around the same young age each time.  Pathology tests proved that this line had a type of platelet clumping disorder in their blood which inevitably led to blockages in major arteries, and resulted in their mortality at around 6 months of age.  The breeders who uncovered these results, have since stopped breeding with this  line, but it is suspected that the line may still be being bred elsewhere.  
  There is currently a line of mice here that have ancestors on one side dating back to the pcd line some generations back.   These mice have been, and are presently being, inbred with no problems showing up so far.   This line will continue to be inbred for a time, and then outcrossed to already proven lines, with close monitoring.


  It never ceases to amaze me how some people think at times.  They seem to think that if they tell anyone about an animal in their rodentry that is sick, then they're going to be blamed for it.  Somehow branded as having a dirty place, or unhealthy lines. The reality is that unless you genetically engineered the organism or gene affecting your pets, in a lab, yourself, then how on earth can you be the cause of it or the blame for it.  
The majority of rodents already have 'bugs'(for want of a better word) in their systems.(ie. mycoplasma) [A lot of conditions in our rodents are endemic]   If their immune systems become depressed then those 'bugs'  have a chance to take hold.  
Something as small as a drop in temperature can be enough to affect a rat or mouse’s immune system just that little bit.  So you see very often a rodent may get a dose of the sniffles seemingly out of nowhere.  

It's only by coming together to help each other out, to treat these problems that we're going to have healthy animals in the long run.  Not by hiding it at all costs, hoping it'll just disappear.    It's also dangerous for someone to hide such a thing as this, especially if there are people coming into their rodentry to look or purchase.  If they aren't told about the illness and/or can't see the affected animals, then they won't know about it, and so may not exercise appropriate quarantine or cleansing methods.  They may then also inadvertently spread the condition around without even knowing they're doing it.
   For someone to ridicule a fellow fancier because they've done the responsible thing in making it known that they have a sick animal at home,  is not only unproductive & spiteful, but a waste of everyone's time.  If a persons reasons for being involved in the fancy in the first place, are not for the benefit, welfare and improvement of the animals involved, in a social, and supportive environment, then they shouldn't be there.

While there are many genes responsible for some of our fancies that are inseparable from the problems they cause,(ie.the Ay gene in mice & manx in rats) these are well known and documented, and appropriate measures taken so as to be able to breed these varieties humanely and effectively.     
   There are a great many more bad genes around that we don't know about and these are the ones that need to be identified and bred out if possible.   Having spoken to many experienced breeders overseas, about the problems arising in their lines of rats and mice.  I feel that we are at a great advantage to learn from our fellow fanciers experiences, in that, because we now know about these conditions, we should be able to avoid them occurring in our own breeding programmes, and/or treat them more affectively should they arise.  

  You only need to drop the word "inbreeding" around a bit and you'll likely have people jumping on you from all directions.   The fact that a lot of the existing problems could have been bred out in the beginning, is a frustrating one.  
   The more breeders who shun this practice, because they don't understand it properly, the more rodents we breed with problems silently passed on down the line, only to be discovered at a later stage when it may be too late to fix.  

  Inbred strains have been developed by laboratories and scientists alike, with positive results rather than negative.
*Gregor Mendel, was reputed to have bred brother and sister mice together for 40 generations with no adverse affects and increased overall body size by over 50% in the process .
*The Sprague-Dawley line was created by mating an original black hooded buck to an unrelated doe, then mating him back to his daughters for seven successive generations. The line was closed. (circa 1958?) which means in effect all this one lab line come from those rats and no outcrosses.
*Many lab strains are closed - if you visit lab webpages, some have been closed from as far back as 1947.  If these rats were suffering any deleterious effects from that much inbreeding they would no longer be a marketable, reliable, commercially viable product for experimentation. The whole idea was to create homogenous rats which give consistency for their purpose.
[Note: while I don't condone the experimentation on any animal, I'm using the above as an example to illustrate the success of, and positive side to, inbreeding.]

  There are always people debating the meaning of inbreeding, so to clarify the terms used, I've included a list below with their dictionary meanings.  
One way we can increase understanding among fellow breeders is if everyone uses the terms for their actual meanings.   
  This same principle should apply with the naming of colours in Rats and Mice.  While I realise that there are colours and varieties out there that have not been genetically described, at least we could all make a start by using the names given to the colours that are genetically described, by the geneticists who described them.   


GENETICS:  The branch of biology dealing with the phenomena of heredity and the laws governing it.

GENETICIST:   A specialist in genetics.

GENOTYPE:   The entire genetic constitution of an individual; also, the alleles present at one or more specific loci.

PHENOTYPE:  The outward appearance of the animal in all of its anatomical, physiological and behavioral characteristics as dictated by genetic and environmental influences.  

INBREEDING:  The mating of closely related animals or of animals having closely similar genetic constitutions.

LINEBREEDING:  Breeding to animals in the same family but not closely related.

OUT-CROSS:  The mating of unrelated or only distantly related animals.  Opposite to inbreeding or closebreeding.

CONGENITAL:  Present at and existing from the time of birth.

CONGENITAL DEFECTS/FAULTS:  Abnormalities of structure or function which are present at birth.  They may or may not be inherited.

HEREDITARY:  Transmissible or transmitted from parent to offspring; genetically determined.

HEREDITY:  The transmission of genetic traits from parents to offspring.  The hereditary material is DNA in the ovum and sperm, so that the offspring's heredity is determined at the moment of conception.

HERITABILITY:  The degree to which inheritance plays a part in the aetiology of a disease.

AETIOLOGY:  The science dealing with causes of disease.

GENE:  The unit of heredity most simply defined as a specific segment of
DNA.  Genes determine the physical(structural genes), the biochemical(enzymes), physiological and behavioural characteristics of an animal.

DNA:  Deoxyribonucleic acid. A nucleic acid of complex molecular structure occurring in the nucleus of cells, as the basic structure of the genes.

LOCUS(plural Loci):  The specific site of a gene on a chromosome.  

CHROMOSOME:  The structure within the nucleus of a cell, containing a linear thread of DNA.  (the individual genes sit on the chromosome)

ALLELE:  One of two or more alternative forms of a gene at the same locus in each pair of chromosomes.  

DOMINANT GENE:  One that produces an effect (the phenotype) in the animal regardless of the state of the corresponding allele.

RECESSIVE GENE:  One that produces an effect in the animal only when it is transmitted by both parents.  (ie. Individual is said to be "Homozygous")

HOMOZYGOUS:  The state of having identical alleles in regard to a given character or characters.

HETEROZYGOUS:  Having different alleles at the one locus.

POLYGENE:  A group of non-allelic genes that interact to influence the same character with additive effect.

LETHAL GENE:  One whose presence causes such anomalies that death is caused prior to birth.  No homozygous anomalous young are ever seen.

TERATOLOGICAL LETHAL:  Produces monstrous young at birth.  Quite often the "Monster" is still-born as a result of various gross defects.  In other cases death usually ensues within a short period.

DELAYED LETHAL: Animal is normal at birth, but eventually dies of a late developing affliction.  The onset may be delayed for months or even years.

SEMI-LETHAL GENE:  (or Partial lethal) One that permits survival under certain conditions.  Death is not an inevitable accompaniment but the individual is always either mildly or severely abnormal.  

SEX-LINKED GENE:  One that is carried on a sex chromosome, especially the X chromosome.

COMPLIMENTARY GENE:  Two independent pairs of non allelic genes, neither of which is functional without the other.  (eg. The variety of "pearl" in rats)

MUTATION:  A structural alteration in DNA present in a mutant that gives rise to the mutant phenotype.

SPONTANEOUS MUTATION: That occurring in nature without the addition of a mutagen.  

SOMATIC MUTATION:  A genetic mutation occurring in a somatic cell, providing the basis for mosaicism.

MOSAIC & MOSAICISM:  The occurrence in an animal of two or more  cell populations, each having a different chromosome compliment.

Dictionary used:  Bailliere's Comprehensive Veterinary Dictionary

A short description of mosaicism.
A mosaic animal is one who's phenotype should not be possible under genetic law.  These freaks of nature, are derived from a mutation in the body cells but not in the germ cells, so this trait cannot be passed on.  
Because of the effects that some genes have upon others (esp. coat colour, eye colour and coat patterning genes) there are certain combinations which are impossible to have, or at least, genetic law would seem to dictate that they should not be possible.
   An example would be a black mouse or rat with black eyes, in combination with either chocolate, blue, dove or a combination of all of these colours on the coat.  If any one of the genes that cause all those three colours were present, then the expression of black should be impossible.
   The unusual combinations of traits that mosaicism creates are not themselves inherited, however there is a possibility that some lines of animals will have a tendency towards mosaicism.   It has been known for a mosaic to produce a mosaic in science, but this was a chance event which did not recur.  
  Even if the germ-cells were involved, the mosaicism itself would not be inherited, only the genes producing the oddity.

  Some examples of mosaicism at work are:  
#A cavy born who's colours were, black, chocolate, red and white.
#The rather unique case of the short hair black rabbit with several patches of chocolate fur and a tuft of angora hair; all in the same animal !  
#Odd-eye  in rats.
#The case of the "tricolour satin mosaic mouse".  This mouse had black &  chocolate on a white background.  

 On a matter of biological terminology, the term mosaic is reserved for non-inherited cases.

I penned this article, in the hope that the above information will help some people to understand genetics and genetic processes a little better.   Yes genetics is an involved topic, one that just isn't some people's cup of tea.   But it's not as difficult as many would like you to think it is.  Developing a basic knowledge of some key factors, to help yourself out in determining which pair of Rats or Mice to breed with to get a particular outcome is a rewarding exercise.   
   I'm happy to chat to anyone about any aspect of rodent breeding, not just genetics, although I do have an avid interest in this.  
Please feel free to email us any time.

  The temperament, health and longevity of "Carawatha" rats and mice attest to this simple FACT.   

Yours in Rodents