The gene pool
In the forties of the last century, the Swedish Vallhund was bred back from a small number of dogs: only 39 dogs (26 females and 13 males) formed the base of today's population. Because of this, the genetic variation within the breed is small and inbreeding is likely to occur. Inbreeding can cause reduced fitness, a higher risk of genetic disorders and reduced possibilities for improvement. Luckily, our breed is still very healthy. Hereditary diseases hardly occur and most animals reach a very high age. But, if we want to keep it like this, it's very important as many dogs as possible are used for breeding.
gene pool
The genetic characteristics of a dog are captured in genes, consisting of one allel from the father and one from the mother. The collection of all those different genes in a population, is called the gene pool. Genes, consisting of two identical alleles are called homozygous, if the alleles are different, the gene is heterozygous.
When a population is closed, which means no dogs from other breeds are added, no new genes will be added to this collection of genes. Through breeding, the number of dogs of the breed may increase, as well as the frequency of particular alleles, but the number of different genes will always stay the same.
By chance (so called genetic drift) or through selection though, for example by excluding animals with particular undesired characteristics, alleles can disappear. This will result in the loss of the variety in genes: the gene pool will become smaller.
the risk of homozygosity
In small populations, inbreeding (mating of related animals) is likely to occur.
Although by inbreeding predictably and uniformity of offspring is improved, (because homozygosity is increased) also
several disadvantages will occur.
Increasing homozygosity causes reduced fitness, the so called inbreeding
depression: a reduced ability of the individuals of the population to survive and reproduce itself. Read more about this here.
Beside this, animals have a bigger risk of carrying two recessive alleles of a genetic disorder. If a harmful mutated gene is
dominant, meaning the presence of only one abnormal allele is enough to make the animal affected, it will probably be eliminated very quickly. But many
genetic disorders are caused by autosomal recessive mutations, which means the disorder will only be expressed if the dog has two abnormal alleles (homozygous). A dog with only one of these
alleles, a so called (heterozygous) carrier, is usually unaffected, but the more carriers of this allele, the bigger the chance offspring receive a copy from both parents and therewith the
disease.
The possibilities for breeders to improve the breed through selection are reduced when homozygosity is high, because selection needs variability.
level of inbreeding
According to the SKV, the level of inbreeding in the Swedish Vallhund is now 27.02%, calculated on 17 generations. When mating for example an average brother and sister, that same level is 25%. This means their offspring would have a 25% chance of getting the same allele from both parents. So, in general, today's Swedish Vallhunds have a bit more genes in common than a brother and a sister.
the Swedish Vallhund compared to other dogs
Graph 1 (and text) and graph 2 were taken from the website of The Institute of Canine Biology. Data were compiled from the information on the MyDogDNA website during March 2016 by Carol Beuchat.
Graph 1
Carol Beuchat, The Institute of Canine Biology
The genetic diversity in the Swedeish Vallhund compared to other breeds. The data for the Swedish Vallhund are graphed in blue. The orange line is for all dogs, the green line is for related or similar breeds. High genetic heterozygosity is to the right, low heterozygosity (= high inbreeding) is to the left.
At Niikes health page you can find her place in this graph.
From these graphs it becomes clear, that compared to other breeds, the genetic diversity in the Swedish Vallhund is low!
Graph 2
Carol Beuchat, The Institute of Canine Biology
This graph depicts the percentage of heterozygosity for different breeds. The red line is the median heterozygosity for all dogs in the database and the green line is the median heterozygosity of mixed breed dogs.
The higher a breed is in the graph, the lower the heterozygosity. The Swedish Vallhund can be found at the orange arrow.
Do you want to know more about this subject? There's a lot of easy to read information on the website of The Institute of Canine Biology .