Understanding inbreeding and the importance of genetic diversity


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 you.

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 desirable dogs.

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 population?

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 rare.

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 recessive conditions?

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 inbreeding.

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 lifespan.

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.

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