When inbreeding is mentioned, it usually conjures up unpleasant thoughts. As with most things, the reality may not be as bad as the expectation.
A close look at the genetic process shows that an animal has two copies of every gene, one each, from its sire and dam. If parents are
completely unrelated, there is no inbreeding. (This is unlikely among animals of the same breed like ISDs.)
Inbreeding is what happens when identical genes are inherited from the parents. For this to occur, the sire and dam must have a common
ancestor. This common heritage is expressed by a term called the inbreeding coefficient (IC) first proposed by Sewell Wright in 1922.
The chance of inheriting identical genes from both parents increases when they are more closely related. When mating two related animals, we
never know exactly how inbred the offspring will be. Only a full DNA analysis could tell us.
However, inbreeding probability can be estimated through pedigree analysis. This number, the inbreeding coefficient, estimates the percentage
of identical genes that are inherited. Inbreeding coefficients are estimates, not guarantees. Inbreeding coefficients are not backed up by real
data. We cannot know exactly which genes are transmitted.
If we mated the same parents many times, some offspring would be more inbred than the estimate and others less inbred than the estimate. The
inbreeding coefficient does not identify whether the genes matching up are desirable or undesirable. If an animal inherits good identical genes,
then inbreeding is beneficial. If an animal inherits bad identical genes, then inbreeding is harmful.
Results of individually mating related animals vary based on the number of identical genes that get matched up by chance, and the quality of
those genes that happen to be identical.
The key to the inbreeding dilemma is to find a balance between genetic selection and control of inbreeding. In fact, inbreeding can be a wise
way to make gains towards the "Breed Standard" in a relatively short time.
The inbreeding coefficient (IC) can range from 0 to 100%. The IC indicates the probability that the two alleles for any gene pair are the same
because they are inherited from a common individual ancestor.
The IC is partially a function of the number of common ancestors in a pedigree. It is also a function of the location of those ancestors in the
pedigree. Are those ancestors great-grandparents, great-great-grandparents, etc.?
The IC is not necessarily a function of the inbreeding of the parents. For example one can mate two highly inbred individuals who are not closely
related to one another and produce a litter with a very low IC. Those individuals come from different families, different inbred families.
(Because the potential number of ancestors doubles every generation, eventually you reach a point where the number of ancestors exceeds the
number of individuals alive at a point in the past. At that point you are bound to find some common ancestors.)
It is also possible to mate two closely related dogs, both of which have low ICs, and boost the IC substantially.
If we had only a single common ancestor to deal with, it would be relatively simple to understand IC scores. However, there are two
1. In the average pedigree there may be a large number of shared ancestors. Therefore, the total inbreeding for a dog cannot generally be
calculated manually. Appropriate software must be used.
2. If there are more than one or two common ancestors in a four or five generation pedigree, the inbreeding is probably already rather high.
Unfortunately, having no common ancestors within four or five generations is no guarantee that common ancestors will not occur in abundance
further back. Some pedigrees of this type (with common ancestors further back than four or five generations) can still achieve moderately high
The number of shared ancestors may be used as a rough guide, as the inbreeding coefficient is very sensitive to when and where they occur in a
The IC measures the probability that a mating will produce puppies with identical alleles of a gene. An IC of 10% states that probably 10,000
of the 100,000 gene pairs in the dog's chromosomes have identical alleles. Inbreeding probably should not be more than 10%. (An IC of
25.0% is equivalent to a brother/sister mating or father/daughter mating. An IC of 12.5% is equivalent to a half-brother/half-sister mating or
As the degree of inbreeding increases, positive traits can be doubled up which is a good thing. However, inbreeding can carry a risk that
defective genes will double up also and produce offspring with genetic problems. Increasingly smaller litters may occur as the IC rises.
Repeated inbreeding combined with popular-sires can cause complete loss of diversity of alleles in a breeds gene pool. This may dramatically
impact the diversity that is so important to breed health.
At the 5-generation level it may appear that a cross is an outcross. Looking at ancestors beyond the 5-generation level may reveal that in reality
that cross is inbreeding.
Improving genetics starts by increasing the percent of good genes in the population and eliminating bad ones. The goal is for consistency and
uniformity. To do this, we make use of the "better" families. As more animals become descendants of those family lines, however, the chance
that we are mating distantly related animals goes up, leading to a gradual increase of inbreeding.
This may not be all bad, because we may have improved overall health and conformity to the "Breed Standard". The challenge is to continue
selection intensity without "too much" inbreeding.
While some inbred matings result in very good dogs, on the average inbred matings perform below expectation due to inbreeding depression.
Thus inbreeding can be a valuable tool but it must be used with caution.
A serious attempt must be made to identify and increase the use of "rare" family lines to promote genetic diversity.