To understand how to improve, or maintain, the health of your
breed, it is important that any decisions you make are based on
accurate scientific theory. If you are unsure why inbreeding
can cause health problems, or why genetic diversity is important,
then the following information has been written especially for
What is a gene pool?
A gene pool is a hypothetical collection of all the variations
of genes in a population. This could be a population of
rabbits in a field, fish in a pond, or dogs in a breed. In a
closed population, such as pedigree dogs, the numbers of gene
variants is unlikely to increase, unless new dogs are brought into
the breed, or mutations occur (which is rare and usually
harmful). A gene pool can, and most likely will, get smaller
when genes are lost through complete chance (i.e. not passed on to
any descendants), or when dogs do not reproduce.
Sometimes an animal having a certain trait can influence how
likely it is to survive and/or reproduce, this could be a faster
rabbit evading a fox, a better camouflaged fish not being seen by
its predators, or a pet dog having a good temperament and being
chosen for breeding. All of these selection pressures can,
over time, shape a population, making some genes associated with
these benefits more common, while others become rarer or are lost
from the gene pool.
How does a breed's gene pool shrink?
Dog breeders will choose carefully and select dogs that possess
specific desirable traits, such as an excellent level of health and
good temperament By applying a selection pressure, (or a
breeding criteria), to a breed, it makes some traits, and the genes
that control them, more common, while others which control less
desirable traits become rarer.
Dogs with desirable traits are likely to be bred from more
frequently, while others that do not possess these traits may not
be used for breeding at all. Over time, the gene variants
associated with these popular dogs become common in the breed,
while those associated with the less desirable dogs may be lost
from the gene pool entirely. These lost genes may include
those that controlled the less desirable traits, but may also
include other genes that just happened to be found in the less
For example, if a longer coat is desirable, then dogs with a
long coat are more likely to be bred from and pass on their
genes. Dogs with a short coat may not be bred from at all and
so will not pass on any of their genes. These lost genes may
include those that produce a shorter coat, but also includes all of
the other genes that contributed to the rest of the dog, i.e. its
eye colour, leg length, quality of hips, temperament etc.
What is the impact a shrinking gene pool can have on a
If a population is made up of 100 dogs and there are 50
different variations of each gene, then the likelihood of finding
two dogs with the same genes is small. If over time the number of
dogs stays as 100, but the number of gene variants shrinks down to
10, then the likelihood of finding two dogs with same genes is much
higher. These dogs will have inherited their similar genes
from an ancestor that featured in both their pedigrees and so they
are, to some degree, related. Therefore, as the gene pool
shrinks, the likelihood of two related dogs mating increases.
The mating of related dogs is known as inbreeding. As
inbreeding increases, so too can the risk of health problems
occurring within the population.
What health problems are associated with inbreeding?
Inbreeding occurs when animals that are related breed.
Many people automatically associate inbreeding with close (or
incestuous) matings, such as a father to daughter mating (which are
banned by the Kennel Club), but this could also include the mating
of more distant relatives. Related dogs are likely to share
similar genetic material, with closer relatives sharing more
genetic material than distant relatives.
Mating two relatives that share similar genetic material means
that their children are expected to be more alike and therefore
have more predictable traits, but this can come at a cost.
High degrees of inbreeding can lead to inbreeding depression
(reduced litter size, increased puppy mortality, reduced fertility,
a shorter lifespan etc.) and an increased risk of developing both
known and unknown inherited disorders.
Why do inherited conditions occur?
A dog's genome (the sum of its genetic material) can be thought
of as a cook book which is split into chapters containing
recipes. These recipes are the dog's gene and the letters
that makes up each recipe is the DNA. Just like a recipe can
be used to make a dish of food, a gene can be used to make a
protein, a building block of a dog's body.
Each dog has two versions of every gene, one that it inherits
from its mother and one that it inherits from its father.
Copies of these variant genes are made by each parent when they
produce sperm or eggs and these are passed on to their
children. When these genes are copied to produce the sperm
and eggs, errors can occur, creating mutant genes (or incorrect
copies of recipes if we maintain our analogy).
Dogs that inherit a faulty gene will make a copy of the error
and can pass it on in turn to their descendants. Just like an
incorrectly copied recipe, the impact it can have will depend on
the type of error made. A spelling mistake of a common
ingredient in a recipe may have no impact whatsoever, while the
changing of a cooking time could have severe consequences.
Similarly a mutant gene may have no apparent effect, or it could
cause a serious health problem.
When can mutant genes cause health problems?
Remember that each dog inherits two versions of every gene - one
copy from each parent. Some health conditions may only appear
if one of the two copies of each gene has an error, while others
can only occur if both copies of the same gene have the error.
A health condition that occurs when a dog has only one copy of a
faulty gene (either inherited from its mother or
its father) is known as an autosomal-dominant condition. Many
of the more severe autosomal-dominant conditions are generally not
passed on to any further offspring because the dog is often too ill
to reproduce, or dies before it reaches sexual maturity. For
this reason autosomal-dominant conditions are usually quite
A health condition that can only occur when a dog has two copies
of a faulty gene (inherited from both its mother
and father) is known as an autosomal-recessive
condition. Dogs with only one copy of the mutant gene are
said to be carriers and are unlikely to show any sign of the
disease, but can pass the gene on to their offspring. The mutant
genes for autosomal-recessive conditions can be the most difficult
to predict, because they can be passed on from generation to
generation without being noticed or identified.
As long as the dog also has a healthy copy of the gene to do its
normal job, then the mutant gene may never be noticed. Often, there
is no way to know that these mutant genes exist, or what they
cause, until they are expressed in a dog with two copies.
Every organism, including dogs and humans, are carriers for many
autosomal recessive conditions which have been passed from
generation to generation without ever being noticed
What is the relationship between inbreeding and autosomal
Dogs that are related to one another are likely to share similar
genetic material. The more closely related dogs are, the more
similar their genetic material is likely to be. This similar
genetic material could be genes associated with positive traits,
but it could also include faulty genes too. The more closely
related dogs are, the higher the risk is that they are both
carriers for the same autosomal recessive conditions. If
these two dogs mate, then there is a risk that the puppies will
inherit a copy of the faulty genes from both parents and will
therefore be affected. This risk of producing dogs affected by
inherited health conditions therefore increases with the degree of
Some autosomal recessive conditions can have a large and
noticeable impact on a dog's health and welfare (e.g. forms of
blindness, epilepsy etc.), while others may only have a very small,
and mostly unnoticeable effect. As the degree of inbreeding
increases, so too does the chance of a dog inheriting more than one
autosomal recessive condition. As the number of these smaller
conditions increase, they can have an accumulative effect, leading
to a decrease in the general health of the dog, otherwise known as
inbreeding depression. This can lead to reduced litter sizes,
increased puppy mortality, reduced fertility and a shorter
Can DNA testing reduce the risk of inbred dogs inheriting
autosomal recessive conditions?
Yes, but only for the condition tested for.
Remember that every dog is most likely already a carrier for
many autosomal recessive conditions. DNA tests are available
for only a small number of the known mutations in dogs, but there
are likely to be many more recessive mutations that we
currently know nothing about.
It is important that breeders DNA test their dogs they are
intending to breed from in order to guard against producing puppies
affected by conditions that are known about. It is also just
as important to take steps to guard against conditions that can not
be known about. The best way to do this is by considering the
impact of inbreeding prior to mating.