How dangerous is inbreeding, really?

This is one of the most common questions I get. And it is also one of the most difficult to answer. Because there are many examples of close breeding, such as sibling mating, mother-son or father-daughter mating and it went fine? The puppies were healthy after all? … So how dangerous is inbreeding really?

I expect that most people who practice breeding are familiar with common inheritance rules and dominant and recessive genes. But I just want to be absolutely clear and have created a so-called “Punnet chart”. Big F = healthy dominant gene and small f = recessive disease gene.

Here is a mother (red) and a father (blue) who both have a copy of the healthy gene (F) and a copy of the sick gene (f). The beige and brown lines show which gene combinations it is possible for the offspring, based on the parents’ genes. I have tried to show that those underlined with red color are from the mother and those with blue color are from the father. Mom and dad are healthy themselves, and you don’t notice anything. But somewhere in their pedigree, they have a common relative who has given them the same disease gene.

Thus, for this combination, 25% of the puppies in the litter will be affected by a genetic disease, and 75% of the puppies will healthy. However, 50% of the puppies will be healthy themselves, but can still pass the disease on to the next generation.

This is generally not a problem as long as the genetic defects are rare. Because then, the probability that two animals that are mated are carriers of the same gene will be small.

This is where inbreeding comes in. When animals are closely related, they will have the same genes, and the likelihood that animals that carry genetic defects will mate is increased.
In a British study of the 50 most popular dogs in the UK, they found 301 genetic diseases, and 71% of them were recessive disorders (as explained above).
The most common disorders were low metabolism and eyediseases. In addition, there are many genetic defects where those affected actually cause the puppy to die in the womb and never be born. These genetic defects will not be noticed in the breed other than statistics, for example that the average litter size is lower than one might expect in relation to the breed size.

These are the breeds where they found the most defects (with the number of defects in brackets).

German Shepherd (58)
Golden Retriever (50)
Boxes (45)
Labrador Retriever (44)
English Jumping Spaniard (42)

The four diseases found in most breeds were hypothyroidism (low metabolism), reported in 43/50 breeds. Then adult onset hereditary cataracts (38 breeds), progressive retinal atrophy (PRA) (35 breeds) and Von Willebrand's disease (bleeding disorder) (26 breeds).
The trouble with inbreeding is that you often do not notice that you increase the incidence of these diseases before they suddenly become a problem. I think it might be worth comparing to a frog sitting in a hot pot that slowly warms up. The frog does not jump out because it does not understand that it is dangerous until the water is suddenly too hot and the frog dies. However, a frog that jumps into a pot of boiling water will try to jump out again as soon as possible. It realizes that it is dangerous because of the danger it suddenly and immediately. Inbreeding is one such pot of water that slowly heats up. Everything works just fine, until it suddenly boils over.

Inbreeding is therefore better seen from a birds perspective. You can not only look at one and one mating, but at all the animals of the same breed to see the general genetic diversity and mating scheme.

Another problem is that it also affects properties that are difficult to "see". You notice that a dog loses sight. But you can also get genetic defects that affect the immune system, fertility, and such general health, also there may not be a single gene to blame, but several genes that play together.

All of these are signs of so-called "inbreeding depression". Low fertility and autoimmune diseases. Also large populations with many animals can have inbreeding depression if only the same animals are left in breeding. Such as "popular sire syndrome" / matador breeding.
The consequences of inbreeding can be seen very clearly in the Norwegian Lundehund (Puffindog). It is not inbred because of human ignorance but simply because of puppy sickness that hit 2 times, which at one point left only 5 live puffin dogs and several of these were also siblings. From these 5, a number of close matings were made which were necessary to increase the number of animals. Since then the population has increased to approx. 1000-1500 animals worldwide, with approx. 50% of these in Norway. 

A study analyzing the Norwegian puffin dog found that the average inbreeding percentage was 33-36%. 33 of the dogs had a breeding percentage of 50% or higher. I like to say that a dog's inbreeding percentage says how much the dog is "related to itself". An inbreeding percentage of 50% is the same as sibling mating. 25% is half-siblings. The Lundehund is somewhere in between. In fact, no matter which puffin dogs you choose to mate, the mating is the equivalent of half-siblings or more.

One also sees the consequences of inbreeding and inbreeding depression in the breed. Inbred depression is especially related to fertility. The average litter size for the Norwegian Lundehund in the period 1990 - 2012 was 2.75. This is very low! In addition, the breed struggles with mating, such as minimal signs of heat from females, behavioral problems during mating (clumsy male dog) and reduced sperm quality. All this is as expected in relation to how inbred the breed is. In addition, the Lunden dog struggles with some genetically related diseases such as IL and gastrointestinal problems. However, there are also many healthy puffins that live long. But the fertility problems are a direct consequence of the inbreeding.

There are also genotyped Norwegian Lundehund, and the results show that all the dogs had no genetic variation on 80% of the genome! it is extremely high! It means 80% of the genes are doubled on ALL puffins and no matter who you pair with you get doubled, No one is only "carrier", no one is "free" ... What do you do?

The study’s conclusion was that the only solution is to cross Lundehund with other breeds to increase genetic variation, and fortunately a project has been started for this. The crossbreedomg project has been going on for some years now, and an unofficial count of the number of puppies in the cross litters and average litter size also shows that there is hope that this can help for the fertility of the Lundehund. I personally know a 2nd generation cross breed (75% Lundehund / 25% Icelandic Sheepdog) and this is a fantastic dog, which most people will think is pure Lundehund in appearance, and which also has the characteristics of Norwegian Lundehund (bevegelie joints, extra toes, closing of ears, etc.). I think those who have been afraid that the crossing will “destroy” the distinctive character of the puffin dog do not have to worry so much and I am incredibly excited to continue following this project.

The puffin dog is an extreme case in relation to inbreeding and for many breeds it will not be applicable to cross into other breeds. Then the most important measure you can take in relation to inbreeding is to think about how many parent animals are used in breeding and how many offspring they each get. Few breeding animals with many offspring each = faster inbreeding. Many breeding animals with few offspring each = slower breeding. This is because the number of parent animals says something about next generation genetic variation. If you have 50 males and 50 females then genetic variation and possible different gene combinations in the next generation will be equal to 50 males 50 females, but if you have 5 males and 50 females then the genetic variation will be limited relative to the 5 males, since all puppies inherit one gene copy of father and one of mother.

All beings that have genes, both plants, animals and humans, have both positive and negative genes. The goal of breeding is that over time you select animals that have genes for things that you want to see more of, such as better hunting instinct, better endurance, better fur, etc., but it is important to remember that one must hurry slowly so that you get the most positive qualities and not so many negative ones. Unfortunately for us, most breeds that exist today are based on much poor breeding done over 100 years ago when the knowledge of these consequences was minimal.

What is so dangerous about inbreeding is precisely that you do not see it until it may be too late.

Published by Maria

Dyreelsker og Doktorgrads-student innen husdyravl- og genetikk Animal lover and PhD-student in animal genetics and breeding

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