When and how does inbreeding occur?

Inbreeding occurs when individuals that are related to each other produce offspring, for example when cousins are mated. Related individuals can be as close as parent/offspring, or as far apart as sharing a common ancestor many generations back in the pedigree (eg. my great grandfather is your mother's father).

Can I incur inbreeding in my animals just "by chance" matings?

Inbreeding can occur by chance or accident, such as when a large flock or herd has many related animals and it becomes too difficult to keep track of the relationships to avoid inbreeding. Close relationships are easier to avoid than more distant ones!

What about line breeding

Inbreeding can also occur "deliberately", because of line breeding or other breeding methods designed to concentrate the genes or traits that are found in a family, or introduced from desirable sources.

What are the problems from inbreeding?

Inbreeding causes 3 main problems:

• Increased faults

  We can't always see easily the "faults" we are trying to eliminate because sometimes the genes that cause these faults don't express themselves in every animal that has them. These traits are called "recessive", and they are difficult to eliminate. If we incur inbreeding, chances are good that we will also accidentally increase these recessive faults, and they will show up more often. Users of inbreeding avoidance software, such as TGRM and X'Aim, say that they see fewer faults after using the software for choosing matings. One such fault is undescended testicles in rams, another is undershot jaws in sheep.

• Loss of genetic potential or "variation"

  When we are trying to improve through breeding, we are selecting individuals that are better (by our definitions) than the average. But doing this, we can only "see" part of their genes as a result of their performance. The "other part" is the result of genetic variation that exists in all individuals. Inbreeding reduces this; for example if all our animals were 100% inbred, there would be NO genetic variation (the animals are all the same genetically), so we could no longer change the characteristics of our flock or herd. There is nothing "new" left and no more improvement possible!

• Inbreeding "depression"

  When animals are inbred, they tend to suffer from "inbreeding depression", often seen as lack of vigour or reduced fertility. We know from studies of different species that as the average inbreeding coefficient increases, individuals become less able to fight disease, tend to become less fertile, grow more slowly and reach smaller mature sizes.

Can inbreeding be avoided?

For the most part, inbreeding cannot be entirely avoided, but it can be managed. We need to understand two basic ideas about inbreeding and how we measure it in order to manage it effectively. These "ideas" are related to how we mate animals and how many of each sex we keep for breeding each year, and we'll discuss them below.

It sounds like a lot can go wrong with inbreeding, how can we manage it?

Because of the risks associated with inbreeding, it is a good idea to avoid inbreeding as much as possible, in the offspring that are created from each mating, and in the future by managing the "related-ness" of the individuals in the flock or herd. These are the two "ideas" about inbreeding we mentioned above.

Tell me about the first "idea" - inbreeding in offspring and the Coefficient of Inbreeding - F

When two related individuals are mated, we can calculate how much inbreeding the progeny will have, and this is called the Coefficient of Inbreeding, Inbreeding Coefficient, or just "F". An individual with no inbreeding has F=0, while a completely inbred individual has F=1. We can either express F as a decimal (eg. 0.05) or we can express it as a percentage (eg. 5%).

This is the standard measurement of inbreeding in individuals, and we can now use it to see if there is a relationship between inbreeding and health, vigour, reproductive ability, disease, and so on. We can also now look at the average value of the inbreeding of our animals. When we do "inbreeding avoidance" mating, we are trying to keep, on average, the inbreeding of the progeny of matings as low as possible.

Ok, what about the second "idea" - the "related-ness" of animals, or Coancestry or Relationship, that has something to do with how many of each sex are kept each year?

We can have individuals that are related to each other, but until we breed them to each other we do not incur inbreeding. For example, if we have a flock of sheep where the ewes are all unrelated to the ram, all their offspring will be related to each other through the ram. But they are not inbred. It is not until we mate them either with each other, or their father or mother, that inbreeding will occur.

The relatedness of the individuals we call "coancestry", and it gives us an idea of how closely related our individuals are. You can see that if we have a more related group of individuals, the chances that we will have some inbreeding increases! Therefore, coancestry is a general predictor of future inbreeding. We can use this to manage future inbreeding.

The more animals we have of each sex, the more options we have when we want to mate them to avoid inbreeding. We can think of these as "family lines"; the more we have, the more we can cross them, and the less chance we have of making matings that result in inbred offspring.

How can we use these two "ideas" to manage inbreeding through mating?

Using both the Inbreeding Coefficient "F" and Coancestry, we can now manage both future inbreeding and the inbreeding of the offspring from our matings. This is exactly what X'Aim does when you request a mating recommendation. We use advanced computer algorithms and the power of artificial intelligence to produce mating lists that equally balance Coancestry and the average Inbreeding Coefficients of the offspring from the mating.