Selection

China

My last article on molecular genetic testing methods was heavily debated here and there. A sentence that was said in this context made it clear to me how much need there is for learning in the field of pedigree dog breeding and why it is so important to rethink our previous selection process, which is often very individually motivated.

Selection

Selection in itself is an essential factor without which evolution is not possible.

We distinguish three types of selection. Natural selection, sexual selection and artificial selection. In modern pedigree dog breeding we are dealing with artificial selection, since this is done by humans. We decide which desirable traits we want to promote or which undesirable traits we want to avoid and select breeding dogs accordingly.

So far we have almost only focused on the phenotype of the dogs. Because that is what we see and what is easiest to change in terms of breeding, or to steer in the desired direction.

Heritability

In population genetics, heritability is defined as the "proportion of genetic variation in a trait over the phenotypic variation of that trait in a population." This can be determined by the so-called heritability coefficient.

The higher this heritability value, the greater the influence of the genes on the desired trait. The lower the value, the lower the influence of the genes and the proportion of environmental influences has a much greater influence.

The phenotype of dogs has a high heritability value and has always been in the foreground in pedigree dog breeding, since we were able to have a direct influence through artificial selection.

Traits with a low heritability value include, for example, allergies and food intolerances. The genotype of the dog determines to what extent a dog can develop allergies and intolerances, corresponding environmental triggers allow the dog to actually have these allergies and intolerances.

In this context, a sentence comes to mind that many dog owners have often heard when they contact their breeder with uncertainty and concern because their dog has suddenly developed an allergy or intolerance. Dog owners are then often told that this has nothing to do with breeding, but is due to incorrect behavior on the part of the dog owner. In the end, of course, it's up to the breeding, because if the genetic trait would not have been present, the environmental factor would not have been able to trigger anything either.

So heritability values are very important when selecting breeding animals.

Selection process

We know how important genetic diversity is to the survival of a breed. If I only select the "best" dogs for breeding, then we inevitably minimize the genetic diversity in the breed, with all the negative consequences that has. One way to keep the genetic diversity at least partially stable would be to only exclude the "worst" from breeding. At this point, I can only quote from Mrs. Sommerfeld Stur's book, which compares the "best" selection to a fine-meshed sieve through which only a few find their way. In contrast, a coarse-meshed sieve that holds back the "worst" but lets everyone else through.

We want healthy and beautiful dogs. So dogs that do not carry any defect genes, meet the standard and meet our own aesthetic standards. So we are not dealing with just one, but with many characteristics that decide whether a dog is suitable for breeding or not. With so many characteristics, a selection process is needed. I would like to explain the two most common selection processes because they are very different.

Minimum performance selection

In most breeding associations, selection is based on minimum performance. There is a limit value for each characteristic which, if exceeded, leads to exclusion from breeding. Each feature is rated equally.

Example: A (fictitious) breed association determines that a Shar Pei must be CNV 2 or 6. POAG/Pll can be free or simple carrier. A maximum of two missing P1s are tolerated in the dentition. He may have a maximum of one HD B hip, no entropion, no wedge vertebrae and must correspond to the breed standard, i.e. must not have any major faults. Maximum size of the dog would be 50cm.

Example Dog 1 is 53cm tall and has a bearcoat coat, a standard fault.

Example Dog 2 is CNV 6 and single POAG/Pll carrier, is missing two P1s and is HD B.

Example Dog 3 had Entropion, is CNV 10 and is missing several teeth, otherwise meets all other requirements.

Dog 1 & 3 will not be allowed to breed with. Due to the breeding exclusion of these two dogs, their genes are lost.

Despite his defective genes, dog 2 is approved for breeding because he meets all the minimum requirements.

This was just one example to understand this selection principle based on minimum performance.

In terms of population genetics, this method is not particularly useful, since blemishes such as an incorrect coat are weighted just as highly as defected genes.

Index selection

Index selection is predominantly used in livestock and zoo animal breeding. Here, all characteristics are considered, evaluated and weighted differently. Likewise, heritability is included, low heritability is weighted higher than high heritability. With index selection, genetic diversity is less reduced.

A phenotypic error such as the bearcoat and size of dog 1 would be compensated by his excellent results in the other areas, his good genes thus being preserved for genetic diversity.

Dogs that are not admitted due to minor faults in the selection based on minimum performance would be admitted for breeding with the index selection, since all characteristics are weighted differently. The advantage for the genetic diversity and the resulting fitness of the breed are obvious.

Rethinking 

In the end, breed associations need to start asking themselves seriously what is more important. Phenotypically optimal dogs that conform to your own interpretation of the standard? Or a population free of known defective genes and a higher genetic diversity?

And this is where I come back to my previous article. Because in order to be able to really assess and evaluate our breed, we have to have the relevant data available openly and transparently. Only then can we determine important breeding values and work out a sensible selection strategy for the Shar Pei. Because phenotype does not correspond to genotype. Evaluation of the phenotype is subject to opinion, interpretation and preference. In comparison, there is hardly any room for interpretation when it comes to assessing the genotype.

I do not want my article to be misunderstood and used as an excuse to breed from dogs that are phenotypically sub-breed standard. On the contrary, I think the standard is very important. Even if the interpretation can be very different due to individual opinions and preferences. It is not okay to breed from dogs that have serious defects without knowing all of the molecular genetic test results available to us today. Because a dog with a serious defect may not be genetically suitable for breeding, or it may just be incompatible with the breeding partner we wanted. Even a show champion that is phenotypically up to standard can be good or bad for breeding, the answer lies in his genes.

I think the Shar Pei deserves that we now reconsider and reassess previous breeding strategies. Because today we have more tools and experts than ever before.

 

Loading comments...