A breeding program implies that we, as breeders, have a goal. These goals must translate into indentifying traits that we wish to change or conserve in our animals.
Traits can be changed or conserved in two ways:
- 1..By selection of the best animals to breed from, and
- 2..By changing the level of in-breeding in the population.
Theory tells us that related animals have similar genes. Thus, for highly heritable traits, breeding with the best animals produces progeny that are most likely to be as good as, or even better than the parents.
For inherited diseases controlled by a single allele, selection will be useful for decreasing, and where possible, eliminating them from the population. In most populations, natural selection is occuring all the time. Animals with a deleterous
abnormality, with low survivability or reproduction, do not leave many progeny. Thus natural selection acts to cull them out.
To fully understand the effects of in-breeding, we need to discuss two other facets of genetics. These are:
Dominant alleles show their effect, even when one allele is present. Recessive alleles are hidden by dominants. They can only show their effect when both alleles in a gene are recessive.
- 1..Dominant and recessive alleles;
- 2..That inbreeding causes the two alleles present in each animal to be alike, more than does any other form of breeding, such as out-crossing or cross-breeding.
While this is the case in many genes, there are others where there is no dominance, and where each allele will show its effect in proportion to its numbers present,ie, 1 or 2 times. If deleterious genes are dominant, or show no dominance, their presence, even in single dose is obvious. Selection can thus act against such genes at all times and act to remove them, even when they are present only in low frequencies.
If deleterious genes are recessive, they will only become obvious if they occur in a double dose at a gene. Thus, recessive, inherited defects for example, remain hidden unless they are present twice at a gene. In many cases, the majority of the recessive deleterious alleles in a population are not seen and can not therefore be removed by selection. This is a general principle that applies to any recessive deleterious gene,
whether it onits own causes an inherited disease, or whether it causes a decrease in a complex, quantative trait such as reproduction. In presently existing populations therefore, deleterious alleles are likely to be recessive, because deleterious alleles, inherited differently, will have already been removed by natural selection.
Inbreeding is the mating of related animals; that is of animals with genes that came from the same ancestors.Of course, such genes may be the products of the duplication of the one single strand of chromosome in an ancestor, and must, in that case, be identical (certainly the same allele). The coefficient of inbreeding is the measure of the degree of inbreeding in an animal. With pedigrees available, and assuming the animals at the back of the pedigree are un-related, it is possible to calculate exact values of inbreeding coefficients for each pedigree.
We know that the animals at the back of the pedigree may also be related. This just means that the calculated value may be lower than the true, absolute value of the inbreeding coeficient. The calculated value is useful for comparing the relative amount of inbreeding that has occured in the generation represented by any pedigree.
Inbreeding, by increasing the number of genes at which there are similar alleles (the genes are homozygous), has the effect of making inbred animals breed truer, ie,more predictably than non-inbred animals. this was used by breeders early last century after Mendels Laws had been rediscovered, to differentiate their breeds by colour pattern, ear shape and so on. Inbreeding therefore got the reputation of being a good thing. It was thought that if you could make cattle homozygous for the genes for fast growth or high milk production, all progeny would then bred true to type for grothw rate and high milk production.
However, for quantative traits such as fast growth rate and high milk production,the inheritance is more complex. In quantative traits, no one can differentiate between genetic and environmental effects. And how many genes must one make homozygous for fast growth rate? Is it 3, 10, 30 or 3,000? merely asking the question shows the impossibility of using inbreeding to fix quantative traits in the homozygous form we want, at all genes that matter.
What has now been realised, is the main effect of inbreeding, resulting from its tendency to pair identical alleles at more and more genes, is to expose recessive alleles. As set out above, if there are any recessive deleterious genes being carried ina population, they remain hidden unless inbreeding causes them to become obvious.
So, while breeders are using inbreeding to make lines breed true in perhaps twenty Mendelian genes concerned with outward appearance, they are casuing thouisands of genes in all other characters to become homozygous, and many of those will cause deleterious effects. In some breeds, specific inherited diseases become obvious. In all breeds there will be a general, often unspecifiable reduction in vitality, so that martalities will rise, reproduction will be lower, and even size will tend to go down.
Can you apprciate the enormous significance of inbreeding in causing problems for dog breeders?