Sunday, November 20, 2011

Recessives and Inbreeding

Continuing on my musings on the silliness that is the "mismark," I would like to present several examples of situations under which "undesirable" markings appear. I plan to do some case studies on certain breeds and why I think some breed standards are completely impractical when it comes to color. To begin, here's some comments on various forms of breeding that involve recessive genes.

One of the most common reasons, perhaps the most common reason why mismarks will appear in breeds is that countless color genes in the canine world are recessive in some way. This makes it quite easy to forget about a gene that was present in the early days of a breed, but disappeared through selective breeding. It only takes one breeding for a recessive to disappear, and if chance favors the production of homozygous dominant individuals and heterozygous carriers, that gene may soon be forgotten. Also, only breeding from the dominant phenotype will lead to production of some homozygous dominant individuals, and eventually a population may end up being mostly made up of homozgous dominant and heterozygous individuals. Of course, since heteozygotes are still in the population, the production of the recessive "mismark" is still very possible. Depending on the percentage of heterozygotes, it may even be almost guaranteed. Remember that if you breed heterozygotes together, you have a 50% chance of producing more heterozygotes (as well as 25% homozygous dominant and 25% homozygous recessive). Even if you take the "undesirable" recessive phenotypes out of the breeding pool, two-thirds of the remaining individuals will still carry the recessive. The likelihood that the recessive will disappear completely is basically zero.

In addition, if there is some sort of recessive being carried by members of a breed, inbreeding will often lead to it appearing at a higher frequency than would be expected if matings occur between more distantly related individuals (see Why Inbreeding Leads to Increased Homozygosity). Remember: all members of a purebred population are relatives due to the small number of individuals that went into starting basically every breed. Often, purebreds have very high inbreeding coefficients, sometimes to the point that, no matter how little relationship two dogs have in their recent pedigrees, they may in fact be as genetically similar as cousins or even closer relatives. So, the more heavy the inbreeding (i.e. the higher the inbreeding coefficient), the more likely that a recessively inherited mismark will appear. If breeders wish to avoid the colors that are seen as "bad," then this is one more reason to add onto the giant pile of reasons to not inbreed.

This silver Labrador is expressing the recessive blue dilution gene (possibly in combination with the liver gene). It is believed by most Labrador breeders out there that the presence of the gene in the breed is the result of a fairly recent out-cross to a Weimaraner. If this is true, the silver Labs will likely be no healthier thanks to the out-cross when compared to regular Labs due to the heavy inbreeding involved in creating continuous lines of silvers. In fact, their health may even be worse than the rest of the breed thanks to that same inbreeding.
One advantage, I will say, that goes along with breeding for a dominant phenotype is there is little to no inbreeding required to get the gene to be fairly consistent. In contrast, the easiest way to get a recessive gene to be consistently produced is very heavy inbreeding. For example, if there is a single dog that appears with a recessively inherited color that a breeder is interested in, and they are the only one of their kind, inbreeding is the only way to guarantee the reappearance of the gene. Let's say a female puppy is born who is an unusual color. As an experiment, she's bred to a male and all of the puppies don't look like her. This means the trait is recessive, and all of the puppies are guaranteed to be carriers. Breeding two of them together would lead to a litter that would likely contain 25% homozygous recessive puppies. If two of those recessive phenotype puppies happen to be opposite genders, then breeding them together would produce an entire litter of recessives! Or, if only a male puppy is produced, he could be bred back to the original bitch (grandmother-grandson) to create the desired litter of recessives. The original bitch could also be bred to her father, since it is quite likely that he was a carrier of the color, and they could produce more like her. However, this is even worse than a brother-sister mating (father-daughter and mother-son matings are the most severe forms of inbreeding possible in mammals). Furthermore, if the desire is to keep the color coming, then only breeding to very close relatives would be possible. Even if out-crosses were done, they would still be relatives and they would still have to be bred back to the heavily inbred original population for the color to reappear. It's creating the breed-within-a-breed I mentioned in a previous post.

These are two white (albino) Doberman pinschers, which are caused by an incomplete form of albinism (making them cream and white with green-yellow eyes). The sort of heavy inbreeding situation I've mentioned happened when the white Doberman appeared on the scene, which is why white Dobermans are far more prone to health and behavioral issues than the rest of the Doberman breed. Sensitive skin is basically guaranteed due to the lack of pigment.

Needless to say, I think anyone who is interested in any "unusual" color in a breed should be very wary. I would say that virtually every breeder that breeds specifically for a color that is not accepted by a breed standard should be approached with extreme care. If the color is recessive, then it is likely that they are involved in very questionable breeding practices. Part of this is that the lack of acceptance from other breeders toward the "unacceptable" color has likely resulted in a very narrow pool of dogs going into creating the lines that they are offering. I reiterate: the narrower the gene pool, the worse off a population will be. If any breeder brags about having a particular color of puppy available, this should be a major warning sign. If that puppy is being sold for a higher price than the "normal" puppies, this is major warning sign number two.

One issue I see with modern genetic research is the possibility that all members of a population may be tested for a particular color gene in an attempt to completely eliminate it. To me, this idea is completely absurd, but I wouldn't put it past some breeders. It should be much more important to deal with inherited health issues rather than genes controlling color, coat type, and other traits in a breed.

3 comments:

  1. Great post Stephanie. I'm a Dobe owner and very concerned about the health of our breed.

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  2. Stephanie, Are there any dog breeds based on a recessive color? I breed Russian Blue cats- a breed obviously based on the recessive color- blue. Actually it takes two genes- Black and Dilute to make a blue cat but I digress. Anyway, I do agree that it only takes on mating for a recessive to disappear. Your last sentence is absolutely right! Why bother coat color alleles when there are much more damaging genes out there. IN cats, HCM is becoming more common in several breeds including mine.

    SC blu Cat lady

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