genetics

GENETICS AND SUCCESSFUL DOG BREEDING

submitted with permission by Marj Brooks
with thanks to Kevin & Donna Frizzell of DeSaix St. Bernards for generously allowing us to use many of their superb array of articles

1. If a purebred dog of breed X mated with a purebred dog of breed Y, both meeting health standards for their breed, is there a better chance the offspring would be healthier than a same breed mating because the gene pool is larger?

In terms of health alone the first answer would be that in breeding two healthy dogs it shouldn't matter if they're the same of different breeds, you're apt to get healthy pups. But this doesn't take into account the question of recessives.

Suppose you breed two dogs of different breeds that both have the same incidence of a recessive health problem. The pups would have the same odds of having that health problem as purebred pups of either breed.

On the other hand, suppose the two dogs were of breeds that have no recessive health problems in common. This would reduce or eliminate the odds of the puppies of having the health problems of either breed. This is the classic explanation for the theory of first generation hybrid vigor. The resulting pups should not be bred though, since they'd have a good chance of having the recessives from BOTH breeds, so their offspring would be inclined to be worse off than the purebred offspring of their grandparents.

Another very important point to keep in mind is that when a purebred carrying a genetic defect is crossed with another breed or mixed breed, the "bad" genes do NOT "go away" even though they may not be expressed in the offspring. If crossed with another dog carrying the same defect, the offspring of that breeding will demonstrate the defect.

Responsible breeders try to identify genetic diseases their dogs might be carrying and to eliminate them by careful breeding. It is ironic, though not surprising, that their efforts to identify and weed out genetic problems have lead some to cry "look at all the genetic diseases purebred dogs have!" A moment's careful thought will lead you to the conclusion that mixed breeds carry the *same* harmful genes (their parents, or their parents' parents, *were* purebreds, after all). The differences are:

1) with some recessive disorders (though not *all* genetic defects) the disease is less likely to be *expressed* (though it can still be inherited by offspring)

2) you have lesser likelihood of ever identifying or eliminating any harmful genes your mixed breed may be carrying.

When you breed two different breeds together, what kind of variation can you expect?
Pfaffenberger's book has some interesting data on this. He did some experiments with four different breeds. They were dogs of approximately the same size, but very different physical appearance and behavior. The results he saw in the first and in subsequent mixed generations are pretty interesting.

What is out crossing?
Out crossing is where the sire and dam are totally unrelated, preferably for three or four generations. The true form of an outcross is between two entirely different breeds because in reality the members of most registered breeds come from a common ancestor (although it may be many, many generations back). It is very rare for out crossed puppies to be very uniform in appearance. Usually there are a very large ranges of sizes, coats, colors, markings, and other distinctive characteristics. This does usually result in a heterozygous litter (the pups have a wide range of genetic material spread among them). This tends to produce healthier puppies, but not always. Also, many of the desired characteristics of the breed are quickly lost (meaning the pups are not usually show quality -- although there are exceptions!). Also, these pups do not tend to reliably reproduce them-selves.

If you are dedicated enough, you can eventually continue breeding by out crossing alone (but don't expect instant or quick results). You should pick dogs that complement each other and are similar in general appearance. This is a long hard road to eventually developing a line. Through out crossing, many health problems can quickly be eliminated (or just as quickly added into your breeding), but usually you do sacrifice some show quality and product ability.

You have to remember that dogs that appear totally healthy may be carriers of genetic problems. To find this out, test mating is done to a dog that is affected with the genetic problem (resulting usually in puppies that are both affected and non-affected carriers) or by inbreeding to a related dog that also doesn't show the signs of being affected (usually littermates are used) this will usually result in some puppies free of the problem, some puppies as carriers, and some puppies affected if both dogs carry the problem gene (this is not as accurate as breeding to an affected dog, but you are less likely to have to put all the puppies down).

There are variations on out crossing. A "true" outcross could be a dog that has totally unrelated dogs bred together throughout the pedigree. This is very rare. On the other hand, "line crossing" is a form of out crossing where dogs from unrelated lines are bred to produce a new line. The sire and dam are usually very line bred from their prospective lines and the resulting puppies are varied in appearance, some looking like the sire's line and some looking like the dam's line and some looking like mixtures of both lines.

How about line breeding?
Line breeding is when the sire and the dam are distantly related: e.g., grandsire to granddaughter, grand-dam to grandson, second cousins, half cousins, uncle to niece, aunt to nephew..... This is the most common form of breeding in purebred dogs. Through this breeding strategy, new genes are slowly introduced and unwanted genes are slowly replaced. This method sacrifices little overall in terms of show quality. Usually the puppies are rather close in general conformation. The only problem with this method is that it often takes several generations to get poor genes out, (or adding desired genes in) resulting in many puppies that have the same genetic problems (or virtues) as their parents. Because some breeders are more interested in winning, they do not place the affected puppies on spay/neuter contracts. This is both a blessing and a curse for the breed. If the breeder is very careful, affected pups can be used wisely to prevent loss of quality, but still remove the affected genes by only breeding the affected pups to known non-carrier relatives. This way the breeder can again try to "edit out" the bad genes. It takes longer this way but less show quality is lost in the process. This process results in dogs that will often reproduce their same level of quality. This is referred to as reaching homozygous litters (more genes of the same kind apparent in the puppies).

Inbreeding and line breeding differ only in degree. Line breeding is less likely to cause harm than inbreeding. Inbreeding is not for novices. Knowledge of genetics and the breed is required for success. For good results it must be well-planned and breeders must be ready for whatever problems it presents.

And Inbreeding?
Inbreeding is where the sire and the dam are closely related: mother to son, father to daughter, sister to brother, half sister to half brother, cousin to cousin. It is the quickest way to find out what poor genes are in the line and what dominant characteristics are in the line.

Although many people are disgusted with the idea of this family incest, it is an extremely useful tool for diagnosing what genes are present. If the genes for bad eyes are present, but hidden or recessive, this will bring them out to their full extent. If there isn't any bad genes, then the puppies will be of very close uniformity and very able to reproduce themselves (theoretically). This is a homozygous breeding. The resulting puppies will have a lot of genetic material that is the same as their parents and grandparents and will be close genetically to each other.

Inbreeding doesn't introduce new genes and does not eliminate bad genes that the line already has. It only shifts them around like a rubix cube. This often results in litters with high show potential. But there are drawbacks. Inbreeding exclusively will eventually lead to infertility. It's like a xerox machine. After so many copies, you have to renew the ink. The same with dogs, you have to introduce new genes.

Inbreeding increases the chance that a gene obtained from the sire will match one obtained from the dam, both stemming from the common ancestor(s) on which the individual was inbred. Thus, inbreeding tends to make animals homozygous rather than heterozygous. The inbreeding coefficient measures the resulting increase in homozygosity. All breeds have a given degree of homozygosity the mating of two dogs from the same breed would not produce a recognizable specimen of the breed!

Inbreeding can increase homozygosity and decrease heterozygosity. So it can duplicate both desirable and harmful traits, both of which can be unsuspected in the line, and may appear. Inbreeding does NOT create anomalies, it brings present anomalies to the surface. Even when the anomalies are present, inbreeding might not reveal them.

An increase in harmful recessives is undesirable but it is not a major drawback if they are identified early. The effect of inbreeding on major polygenic traits is greater. Generally, traits that are highly inherited (ie largely additively controlled) are not adversely affected by inbreeding but, traits under non-additive control, especially those tied to dominance and thus not of high heritability, are often markedly harmed by inbreeding.

An inbred female produces outbred offspring when bred to an unrelated sire and an outbred female whelps inbred progeny if bred to a related sire.

OK: How Do Pedigrees Figure Into This?
Remember that it is difficult to spot unaffected carriers. When an affected dog shows up, its pedigree is often examined for likely carriers. For example, Progressive Retinal Atrophy (PRA) is a common problem in many breeds. There are dogs that come down with PRA that have a certain ancestor in common. That ancestor may then be considered a possible carrier and line breeding on him is avoided. This is a simplistic picture, obviously, since it's possible for an unaffected non-carrier of PRA to come from an unaffected carrier that came from an affected dog (therefore the affected dog is in the unaffected dog's pedigree). If a blood test is ever developed that shows the presence of the recessive in an unaffected dog, then much more accurate breedings may be done.

There is rarely only a single problem a breeder is trying to screen for. Suppose a suspected carrier of PRA is known for producing excellent hips. A breeder might therefore introduce that bloodline into theirs for the hips, and be willing to have the possibility of PRA show up in the line. In screening out one problem you might have to accept the possibility of another appearing.

Examining the pedigrees also lets you know what percent of ancestry the dogs share (since the relationships are often much more complex than simply cousins or aunt/uncle, the degree of common ancestry is often given as a percentage instead) and decide whether or not it's acceptable given your current goals.

What Are Like-to-Like Matings and Compensatory Matings?

Like to Like: This mating implies the best to the best and the worst to the worst where the worst is not used at all. For most breeders, like to like matings are between dogs which resemble each other greatly and so similar type dogs are bred. These dogs may or may not be closely related.

The pups resemble their parents because of the genes in common with them. If those parents resembled each other their progeny would be even more like their parents. This tends to make the population look more uniform, however there is little increase in prepotency from this technique.

Compensatory Mating: This unlike to unlike mating is used by breeders to correct a defect in an animal by mating it to another animal that might correct the defect. The system is basically simple but the breeder must identify faults and virtues and it requires breed knowledge. The pedigrees of both dogs should be examined carefully to try to identify the ways in which the dogs differ and what the expected outcomes could be. A correct dog and not one who errs in the opposite direction is required. That is, if you want to improve structure, look for a dog with correct structure and not an overbuilt dog. This technique often results in only one or two pups with the combination desired.

But this is all very vague and complicated!
Yes, it is. There are no easy answers, and there are different things to consider in every breed. This uncertainty with respect to genetic inheritance is exactly the reason that breeding is so difficult to do right. It helps immensely to have a "mentor", someone who is familiar not only with the breeds, but the lines your dog belongs to -- advice from such a knowledgeable person is often extremely valuable.

If we knew everything about genetics, we wouldn't have problems with our dogs any more. We'd eliminate Hip Dysplasia, PRA, heart problems, thyroid problems, seizures, etc. within a few generations if we knew everything.

Unfortunately it's an art that few people are actually very good at.

reprinted with kind permission from Judy Bohnert

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