The Coefficient of inbreeding
The inbreeding coefficient is defined as the
probability that both genes (alleles) of a pair (one inherited from each of the
parents) are identical by common descent. In other words they both originate
from a single allele, from a single ancestor within the pedigree. It occurs when
particular ancestors appear on both the sire and dam sides of a pedigree. The
degree of inbreeding depends on how many common ancestors there are and how
close they are to the dog itself. For example, a common grandparent has a very
much larger impact on the degree of inbreeding than does a common great great
grandparent. The degree of inbreeding is computed using a formula known as
Wright’s Coefficient of inbreeding, first published in 1922 by the great
American geneticist, Sewall Wright.
The value provided here is calculated using 10 generations (2046 ancestors) but
to understand what the values mean consider this: If we just look at a single
generation mating and ignore any previous inbreeding the following matings
produce inbreeding coefficients of:
Brother x Sister 25.00%
Parent x offspring 18.75%
Grand parent to grandchild 12.50%
First Cousin 6.25%
From the published data provided by the Kennel Club, 137 IWS litters have been
registered from January 2002 to September 2009.
The distribution of the COIs is shown in the table below and are calculated to
10 generations (2046 ancestors)
|
COI |
No of litters |
% of total |
|
≤ 5% |
10 |
7.3 |
|
5.1 – 10.0 |
31 |
22.6 |
|
10.1 – 15.0 |
36 |
26.3 |
|
15.1 - 20 |
42 |
30.6 |
|
20.1 - 25 |
13 |
9.5 |
|
≥ 25.1 |
5 |
3.6 |
Why is minimising inbreeding so important.
Most canine genetic diseases (75% +) require two copies of
a faulty gene (one from each parent) to be inherited in order to cause the
disease in the individual. A carrier, where only one copy is inherited won't
show the disease but could pass the gene on causing disease in subsequent
litters. Inbreeding increases the chances of this happening
and continued inbreeding greatly amplifies this possibility. The same applies to
polygenic diseases, such as hip dysplasia, where more than one gene is
responsible for the disease. In the natural world, Darwin's theory of evolution
by natural selection would come into play meaning that any gene mutations
which cause an individual to have a disadvantage would either disappear
completely or be present in the population in very small numbers (survival of
the fittest).
Unfortunately, in pedigree dogs this natural control mechanism is not allowed to
exist because man selects the matings and he is not as discerning as nature.
Consequently, although low COIs are important for minimising inbreeding
depression, they do not guarantee that any given genetic disease will not occur
in dogs. Indeed, it is possible that for a given disease, matings with low COIs
could be just as harmful as line breeding for spreading the deleterious genes
around.
However, as there are no genetic tests for any of the genetic diseases known to
exist in the IWS and breeders in particular are not forthcoming about divulging
which of their dogs have developed which disease, it is currently very difficult
to eradicate genetic disease.
In this situation, the best that can be done is to try to mimic nature and have
litters with low COIs.