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Breeding Information
Genetic Diversity - the Dark Side of
Inbreeding
Catherine Marley
If
you can imagine building a house, there are numerous subcontractors working on
it, the framer, the electrician, the plumber, the bricklayer etc. Now each
worker gets two sets of plans for his part of the job. Of course there are a
number of other sets of plans or blueprints for this kind of job back at the
main office, but for the job in this house, the worker only gets two of the many
possible plans.
A gene is like a long blueprint given to the subcontractors in the body.
If there is a mistake in the blueprint, the worker either does the job
incorrectly or he doesn't finish the job. All the possible plans (or genes) that
one might use for a particular job are termed alleles. In this case, the
worker gets two sets of plans (genes) of the many possible ones (alleles), and
he puts one set in each hip pocket. The pocket is the analagous to the "gene
locus". The locus is an actual location on each of a pair of chromosomes in
the cell. Each cell has multiple pairs of chromosomes, - the number is species
specific. Each chromosome contains many loci for all the jobs that have to be
done in the body. There is one allele, specific for each job, residing at the
identical locus on each of a pair of chromosomes, - just like the two sets
of blueprints in the worker's hip pockets. The animal got one set of those
paired chromosomes from each of it's parents. In the carrier state of a
recessive genetic mutation, the animal has gotten one good copy of the gene in
question, and one defective copy. In this case, one good copy is sufficient
for the work to proceed and the animal to be healthy.
Genetic diversity in a population means that the population contains most
of the possible alleles (alternate sets of plans) for a particular gene locus
rather evenly distributed throughout the population. (Of course an individual
animal can only have two of those alleles, for every gene locus, in his or her
private collection.) This is where you start getting into trouble in an isolated
population, such as in quarantine countries, or in very small countries. A very
popular stud arrives in the country. Soon every bitch in the population is bred
to him. Suddenly every puppy in the country has one or the other of his
alleles for every gene locus. By breeding every bitch to the one male, you
have selected AGAINST all the other alleles in the adult male population.
These alleles will disappear unless they are handed down to offspring. This is
genetic death. (death before having a chance to reproduce). Genetic diversity
has been lost.
Let us suppose that there were 200 males, and 200 females in our hypothetical
breeding population. But all 200 bitches were bred to Mr. Wonderful. Now,
instead of having 200 different paternal gene collections represented in those
200 litters, we have only one. Now someone says, "We got such great type from
this dog, let's breed his daughters to him as well." Now you are throwing away
another whole set of alleles.
Now suppose this dog carries a hidden defect. In the first 200 litters sired by
Mr. Wonderful, one half of the pups are carriers of that same gene. In selecting
for the gene set that this dog carries, you have selected FOR the defective
gene, and AGAINST a "good" one which may have been in the population before
Mr. Wonderful came along.
Lets say that by now, (which is usually the case in popular sire effect), the
overall carrier rate for the bad gene is 50%. This means that every other
animal in the population carries at least one copy of Mr Wonderful's defective
gene. The chances are 1/2 that any dog carries it and 1/2 that any bitch
carries it. The chances are 1/2 x 1/2 or one in 4 that two carriers will be
mated, even if we bred them randomly. But by now Mr. Wonderful's "look" has
become a showring necessity. In all likelihood, we will be selecting, for our
breeding, that dog and bitch who look most like Mr. Wonderful. The very plain
pair have been neutered and placed a pets.
Again we have selected for the genes of Mr. Wonderful and against
the other genes in the original population. But the pair resembling their famous
sire are the very ones most likely to be carrying the heaviest helping of Mr.
Wonderful genes, both good and bad. So the chances are that more than 3 out of
four litters will produce carriers, and one out of 4 litters will now
have affected puppies. In each of these affected litters, 3 out of 4 pups
will carry the defective gene. Only one in four pups in each of these litters
will be "clean".
What is the solution? The most obvious response is "don't breed any dogs
carrying that gene". Now suppose we decide we will not permit breeding of
affected dogs or carriers. Abruptly, we eliminate from our breeding the the 50%
of the population which carry the defective gene. Which dogs are most likely to
be carriers? The ones that look like Mr. Wonderful, of course. What is now left
in this genetically decimated population? Precious little, perhaps not enough to
go on with. There may be genetic problems that have surfaced because of the lack
of "good" genes in the population, neonatal losses, failure to thrive,
allergies, etc.
What happens, on the other hand, if we outcross to a line which has never
had this problem? Suppose we assemble a number of unrelated dogs, - lets call
them all "Mr.Clean" - and breed each of our bitches to a different Mr. Clean.
Again 1/2 of the bitches which are bred to our Mr.Cleans, are carriers. This
means that they have one "bad" copy of the gene, and one "good" copy. Since the
sires have only good copies of the gene, half of the pups from the carrier
mothers will be carriers, but NONE will be affected. In a late onset
disease, identifying affected animals before they are bred will be a problem,
but testing for the disease before breeding will reduce the number of litters an
affected animal is likely to have. By using mainly outcrosses, even the
accidental use of affected dogs will not increase the prevalence of affected
offspring.
What about other problems? If this group of sires carry undesirable traits, they
will each have a different collection, - alleles different from the inbred
population and from each other. The likelihood of two "bad" copies of any gene
getting together is thereby markedly reduced. And this infusion of new genes has
introduced new genetic diversity into the impoverished line.
Put these two approaches together. Let's say we do not intentionally breed
any animals who carry two copies of the defect, and we only use known
carriers when there is a good reason to do so, and then ONLY to unrelated
animals. This approach allows the gradual removal of the defective gene from
the population, and the preservation of most of the good qualities of good old
Mr. Wonderful. It restores a healthy degree of genetic vigor to the population,
and lessens the likelihood that other "bad" problems will manifest themselves.
This is what the "genetic diversity" approach to breeding is all about. The
"wholesale genetic slaughter" method may be appropriate in a genetically diverse
population with only an occasional individual case of the disease in question.
(Of course you rarely have this kind of problem in a genetically diverse
population.) You get into the heavily "loaded" state only by inbreeding. Using
the "wholesale genetic slaughter" method, even if it were practically possible,
will not cure the problem in this inbred population, because the real culprit
is not the defective gene, but the inbreeding. Besides you have to wonder
what you will have left when you are finished. The dogs are still inbred, with
all the problems that go along with inbreeding. In fact, they are twice as
inbred as when you started, because you threw away half the genes. You won't
have much of the genetic disease you select against, but maybe you won't have
dogs either.
A reasonable course of action in any genetic disease demands that
attention be paid both to removing the gene where possible without seriously
degrading the genetic viability of the population, and taking steps to provide
increased genetic diversity if it is needed.
Email
Catherine Marley
___________________________________
HOW ARE DEFECTS INHERITED?
The magic of
heredity - DNA, chromosomes, genes
All animals are made
up of billions of tiny cells. The nucleus of these cells contains all the
information to regulate the activity of the cell and therefore the form and
function of the particular body tissue, and ultimately to form the individual
animal. This information originally comes from the parents of the animal, with
approximately one half from the mother and one half from the father.
This information from
which all life develops is in the form of DNA (deoxyribonucleic acid). A gene is
a portion of a DNA molecule, carried on a chromosome. Think of a chromosome as a
long string of genes. Hundreds and even thousands of genes may be carried on a
particular chromosome. Chromosomes occur in pairs in the cell nucleus, except in
the egg and sperm where they occur in half pairs. When an egg is fertilized by
the sperm, the resulting cell from which the animal will develop has complete
pairs again. This is the way in which one half of the genetic information comes
from each parent.
The dog has 78
chromosomes, in 39 pairs, on which approximately 100,000 genes are located. This
makes up the animal's genotype. The phenotype is what you actually see in the
animal, and this can be influenced by both environmental and developmental
factors. For example, a dog's size as an adult is determined partly by his or
her genes, but is also influenced by environmental factors such as its health as
a puppy and the food it eats.
Each gene in a
chromosome pair has a partner at the same position (or locus) on the matching
chromosome. Each member of a gene pair is called an allele. A gene can have many
alleles within a population but an individual animal will have only 2 alleles
which influence a particular trait. If the 2 alleles are identical (AA or aa for
example), the individual is homozygous at that locus; if the alleles are
different (Aa), then heterozygous.
If the allele is
dominant, only 1 copy is required to express the trait; if recessive
then 2 copies. Upper case letters are traditionally used to represent dominant
traits, lower case letters for recessive traits. Thus for a dominant trait,
either AA or Aa will express the particular characteristic, while for a
recessive trait only aa will express the characteristic. The heterozygote (Aa)
will be a carrier - clinically unaffected but able to pass the harmful allele to
the offspring.
Example:
Progressive retinal atrophy
(PRA) causes blindness in many breeds. P represents the dominant allele, and p
the recessive allele. Since PRA is a recessive trait, p is the affected allele,
and P the normal allele.
The genotypes PP and
pp are homozygous. Dogs with the genotype PP have normal sight and those with pp
are affected.
Pp is heterozygous.
These animal have normal sight but are carriers. They will pass the allele for
progressive retinal atrophy to approximately half their offspring.
Phenotypically, both
PP and Pp have normal sight, but their genotype is different. At this time, as
with most recessive disorders, there is no way to identify carriers (animals
with the genotype Pp) until affected offspring are born.
Sex-linked characteristics
are slightly different. Females have a pair of X chromosomes (XX) while males
have 1 X and 1 Y chromosome (XY). Thus 1 dose of a recessive X-linked trait (x)
will cause the expression of that characteristic in a male, while a female with
only 1 dose(Xx) will be a clinically unaffected carrier . The bleeding disorder
hemophilia is probably the best known example of a sex-linked condition.
Defects: inherited
or not?
A disease condition
or abnormality may be caused by many factors. Some of these are genetic; that is
the disorder is a result of a mutation in a gene that carries particular
information. Some mutations are spontaneous, such as a mutation caused by toxins
consumed by the mother during pregnancy. An inherited defectis one in which the
defective gene has been inherited from one or both of the parents.
Many conditions that
have a well-documented hereditary basis may also have other causes. For example,
there are several forms of hereditary cataracts, but cataracts may also occur as
a result of injury, toxins, or a disease such as diabetes. In trying to
determine whether a disorder is inherited, your veterinarian will look at many
factors, including the age the disorder becomes evident, whether littermates or
other relatives are affected, and whether the defect is known to occur in that
breed. It is very important that inherited disorders be identified so that
information can be relayed back to the breeder, and on a larger scale, so that
breeding programmes can be designed to reduce or eliminate these debilitating
conditions in dogs.
Back to top
Patterns of
inheritance
The specific pattern
of inheritance has not been established for many of the disorders that are
believed to be inherited. Where the mode of inheritance is not known, breeds
that have an increased risk relative to other dog breeds are said to have a
breed predisposition for a particular condition.
The following
describes known patterns of inheritance.
Autosomal dominant
Only 1 copy of the gene,
which may be inherited from either parent, is required to produce the trait. The
parent with the dominant trait will pass the affected gene to approximately half
its offspring, and the trait will be apparent in both the parent and the
affected progeny. These conditions are uncommon because, as long as it is of
early onset (ie becomes apparent before breeding age is reached), the disorder
can be readily eliminated by avoiding the breeding of affected individuals.
In many instances
however, there is incomplete dominance. The trait may be dominant with
variable expressivity, which means that if either parent is affected, all
puppies have a susceptibility to the disorder but not all will be affected
equally. Alternately, a dominant trait may have incomplete penetrance. If
penetrance is 75% for example, only about 3 quarters of the pups who inherit the
trait will express it.
Autosomal recessive
This is the most common mode
of inheritance for genetic conditions in dogs. Progressive retinal atrophy
(PRA), which causes blindness in many breeds, is such a trait. To be affected,
the animal must inherit 2 copies of the gene (genotype pp), 1 from each parent.
Dogs with the genotype PP (normal) or Pp (carrier) will be clinically normal but
the carrier will pass the affected gene to approximately half the offspring. As
long as carriers (Pp) are mated to normal animals (PP), the offspring will be
unaffected but some will remain carriers. If 2 carriers are mated, some of the
offspring (approximately 25%) will be affected.
example:
As long as the
frequency of a gene for a recessive disorder remains low in the population, the
particular gene may be passed along for many generations before by chance 2
carriers are mated and affected individuals are born. However, the gene
frequency may become unusually high due to breeding of close family members, or
because of the "popular sire" effect , where a sire with a harmful recessive
gene is mated frequently because of desirable traits.
Because the recessive
gene is carried in the population in outwardly normal animals, it is very
difficult to eradicate these traits. However the incidence can be reduced by
identification of carriers through test matings or through various tests that
have been developed, and the conscientious use of this information in breeding
programmes. Veterinarians, dog breeders, and breed associations must all work
together for substantial progress to be achieved.
Sex-linked traits
In these traits,
the gene is located on the X chromosome. Males have 1 X chromosome from their
mother, and 1 Y chromosome from their father, which carries little information
other than maleness. Females have 2 X chromosomes, 1 each from their mother and
father. So if a mother who is a carrier for a harmful recessive gene (Xx) passes
the recessive gene (x) to her daughter, the daughter will be an unaffected
carrier, but her sons who receive that gene will be affected.
The bleeding disorder
hemophilia is the best known of the X-linked traits, which are uncommon in the
dog. Control programmes are possible because carrier females can be identified
through blood screening.
The
above-mentioned traits are inherited in a straightforward manner. Many others
are inherited in a more complex fashion. In fact, most traits that are selected
for in the dog are the result of the interaction of many genes. Modifying genes
may influence how other genes are expressed. As mentioned above, a trait may be
dominant, but with incomplete penetrance so that it is not always expressed.
Epistaxis occurs when alleles at one locus mask the action of another pair
of alleles.
Polygenic inheritance
Polygenic traits
are controlled by an unknown number of genes. The gene expression is influenced
by a variety of factors including gender, nutrition, breed, rate of growth, and
amount of exercise. These traits are quantitative traits - that is, there is a
wide range within the population. Such traits include height, weight, character,
working abilities, and some genetic defects. Heritability varies within
different breeds and within different populations of a particular breed.
Because it is virtually
impossible to determine the exact genotype for such traits, it is difficult to
control defects with a polygenic mode of inheritance. The best attempts at
control are based on a grading scheme for identification of the defect and a
breed policy of recording and publishing the results for as many dogs as
possible. Canine hip dysplasia is a polygenic trait that remains a
problem in most large breeds of dog, despite efforts to control this condition
dating back to the 1960s. Breed organizations and veterinarians in various
countries have developed control programmes that rely on radiographic evaluation
and a central registry of dogs. Thoughtful selection by breeders, using this
information, has greatly reduced the incidence of hip dysplasia in those breeds
in particular countries.
Back to top
This database is funded
jointly by the
Animal Welfare Unit
at the
Atlantic Veterinary College,
University of Prince Edward Island,
and the
Canadian Veterinary Medical Association.
Copyright © 1998
Canine Inherited Disorders Database.
_______________________________________________________________________
History
of Breeding Strategies
by Melissa E. Parsley,
Parsley Glen
Whenever two breeders get together they are almost certain to speak of
pedigrees. Yet surprisingly few will have a clear idea of what is meant by the
pedigree and what alternative breeding methods exist. This article will endeavor
to tell in simple terms what is meant by "inbreeding," "outcrossing" and "linebreeding,"
and to show how these methods can be used in your pedigree to map out a breeding
program that can improve your stock. With care, your pedigrees can become a map
to guide you where you want to go, to get the look you wish to see consistently
produced and the soundness upon which to base your line.
For centuries the animal breeder depended upon the method of survival of the
fittest to guide his breeding plan. Few had the knowledge and wisdom to handpick
the best breeding pairs for improvement of their herds. A notable exception was
the biblical herdsman Jacob, who was born in 1858 B.C. In a dream he was taught
that certain principles of genetics, and not superstitions, were responsible for
the amazing success in his breeding program. Apparently the solid-colored sires
were producing spotted offspring due to hereditary factors carried in their
genes for generations (and according to the laws of genetics later discovered in
scientific study by the monk Gregor Mendel). The solid-colored sires were
hybrids, built up from generations of crossbreeding between spotted and
plain-colored ancestors. God explained to Jacob that, hidden beneath the
plain-colored coats, certain of the males were actually spotted sires. Jacob
made a fortune with this gem of genetic insight.
In the 1900s science began to assist animal breeders. Mendel (1822-1884) had
defined certain rules of genetics (and anyone today can find the outcome of his
studies in any eighth-grade science book in plain and simple terms). However, it
was the animal breeder Robert Bakewell who would put these laws to work on
domestic animals and claim for himself a well-deserved fame as not only the
foremost breeder of the world, but also as the "Father of Pure Breeding." Born
in Leicestershire, England, he earned this title by transforming the important
classes of domestic animals into the producers we know and trust today. He used
the method of "close breeding," i.e., the breeding in family
lines. All modern breeding theories are based upon Bakewells’ work.
The two basic laws he discovered are: (1) "Like produces like," and (2) "No two
animals are ever identically alike." Because of the first law, a breeder can
stick to a family line of dogs, breed from fairly close relationships of animals
possessing the required qualities and quickly build a strong line. This method
is called linebreeding. Linebreeding is considered to be both fast and
effective, but it holds certain dangers for the unwary.
Because of the second law, we notice a variation in our animals. Therefore, a
process of selection is necessary (and that’s where your responsibility as a
breeder comes in), but it is also effective. In other words: Human choice is
responsible for both improvements and the variations in type that we see in our
breed. Many breeders use the method of "outcrossing" in which they choose the
best to breed to the best, regardless of relationship. This is a safe method but
slow.
The third method is that of crossbreeding. The variety in type derived from this
method will be difficult to standardize into its own breed. First-crosses only
work where the qualities most desired are dominant or where the two breeds
originated from a common source. Such breeding should be undertaken only by the
breeder who will devote his full lifetime to the project, who has a good working
knowledge of Mendel’s laws on genetics, and who can keep firmly in mind his own
ideal and continuously tighten his program along such an ideal. ("Dogdom" is
filled with good examples—one that comes to mind is the Jack Russell terrier.)
Inbreeding was the basis of Blakewell’s success. It consisted of the mating of
mother to son, sire to daughter, or brother to sister. Inbreeding will not of
itself create superior specimens from inferior stock, for the proper factors
must be present in the first place and the best quality must be the basis for
your work. But, because bad qualities are inbred with the good, it takes
vigorous selection on the part of the breeder to eliminate the bad
characteristics by rejecting them. Inbreeding, when based on a prime foundation
and practiced through vigorous selection, is one of the most reliable means of
purifying a breed. Best of all, the resulting offspring will have, through
process of elimination, a prepotency backed by the most powerful of hereditary
influences—obtained because of the simplicity and strength of the ancestry.
However, unless the breeder recognizes a good dog from the particular breed when
he sees one and possesses the right stock to start with, inbreeding can bring
disaster as the breeder tightens in bad points. Most purebred animals have a
written Standard, which should act as the bible for a breeding program,
dictating what is acceptable and what is not. Every breeder should have studied
the Standard until the word-picture forms a vivid vision in his mind—a vision
beside which he stands each animal he produces in his program. Today we have the
added benefit of the Illustrated Standard by Jean Simmonds and articles
in our breed magazines to help us develop our eye. However, it takes more than
just reading the Standard now and again—it takes a concerted effort and a true
study.
Another point to consider is the necessity of selecting your stock for more than
their show potential. Unfortunately, many of our champions are not suitable
breeding dogs as it is equally important that their potency and potential for
reproduction be carefully guarded. Also, domestic animals must be easy to handle
and possess a manageable temperament. When you select for vigor and fertility,
as well as temperament and the sound structure called for in your breed, the
future of your lines will be guaranteed.
The difference between linebreeding and crossbreeding is in the degree. Whereas
inbreeding mates father to daughter, and mother to son, and brother to sister,
linebreeding begins with mating half-brother to half-sister followed by mating
of uncles to the resulting offspring (or aunts or cousins of all kinds in a
close selection of lines). The development of a pure line will eventually occur.
The line should be superior to the average in the breed. A linebred pedigree is
valuable or dangerous in exact proportion to the quality of the individual dogs
from which you have selected. Linebreeding will give you a wider range of choice
within your breed.
So, to review, both good and bad hereditary characteristics are present in
various degrees in every animal. For improvement, we wish to secure and retain
the desirable characteristics. This can be best accomplished through inbreeding
and, to a lesser degree, by linebreeding. Using these methods, undesirable
traits can be bred out and desirable traits can be bred in, while always keeping
in mind that your end product can only be as good as the foundation it was built
on. As Lloyd Bracket, the father of the German Shepherd breed, put it: "Physical
compensation is the foundation rock upon which all enduring worth must be
built."
First appeared in the July/August 1994 issue.
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