What is 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.
The pros and cons of mating related dogs
Mating two relatives that share similar genetic material means that their children are expected to be more alike and therefore have more predictable traits, for example mating two Labradors together will produce offspring that are Labrador shaped, while mating a Labrador to a Poodle can produce a range of different offspring. Although producing puppies with more predictable shapes may be beneficial, close inbreeding 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.
What is the relationship between inbreeding and simple inherited disorders?
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 is known as Identical by Descent. 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 (a health condition that can only occur when a dog has two copies of a faulty gene - inherited from both its mother and father). 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.
What is the relationship between inbreeding and complex inherited disorders?
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 cannot be known about. The best way to do this is by considering the impact of inbreeding prior to mating.