Kallylace Cattery


Line breeding: The Good, the Bad...and the Downright Ugly


At Bydand, we have always believed that the way to produce consistent quality is by selective linebreeding, and we have employed it as a breeding system successfully for many years.
(At right is a Bydand Silk displaying the results of a classic Brackett's Formula linebreeding. Predictably, this dog strongly resembles the dog off whom he was linebred.)
Because lot of people do not understand linebreeding, we thought we’d explain it in some depth for those interested in learning more about it.

The best way to begin is by defining the terms as used by breeders: 

Inbreeding is a systematic program of breeding closely-related animals- to breeders, this generally refers to father/daughter, mother/son, and full sibling matings.

is a term used only in animal husbandry to describe a less- intense program of inbreeding- usually uncle/niece and half-sibling matings.  In breeders’ terms, one linebreeds off a specific animal, and that particular animal may show up multiple times in a pedigree.

is specifically the breeding of two unrelated inbred strains within the same breed, and typically produces hybrid vigor (in what geneticists refer to as the F1, or first-generation, hybrids), but the resulting animals generally do not reproduce themselves with any consistency, unless bred back into one of the original strains.

is the repeated breeding of unrelated animals, with no common ancestors for several generations in their pedigrees, and cannot really be classified as a breeding system, although it is widely used by both novices and commercial breeders.

Hybridization  in dogs generally refers to the deliberate crossing of two different breeds. This typically produces heterosis, or "hybrid vigor", where the average performance of the hybrid exceeds that of the average of the parent breeds. A first-generation hybrid is
called an F1. An F1b is the result of a backcross of an F1 hybrid with a dog from one of the original "parent" breeds. An F2 hybrid is the result of a pairing of two F1 animals. 

A lot of people are frightened by the terms linebreeding and inbreeding, and assume that the use of these breeding systems inevitably results in genetic problems.


In fact, the opposite is often true. By utilizing selective inbreeding, lines can be developed that are genetically free of undesirable recessive traits. Constant outbreeding, on the other hand, produces a situation where those genes remain hidden until they are widely dispersed throughout the population, at which point animals displaying the undesirable trait begin to appear everywhere in the breed, and there is little hope of selecting away from it.
Breeds that become extremely popular in a short time often fall prey to widespread health and temperament problems, and it is usually erroneously blamed on "too much inbreeding", when in fact it is usually a result of constant outbreeding, with little or no health screening performed on the breeding stock.

So why has inbreeding, and its less intense form, linebreeding, gotten such a bad rap?
It’s simple. We are a society with a strong social taboo against incest, and we carry this taboo over into our feelings about animals. Nature, on the other hand, has no such reservations, and many wild populations are by definition inbred (especially in species with strong social orders) as generally only the dominant males breed.
What about inbreeding depression? In other words, how long can one inbreed before the bottom falls out? Geneticists have studied this both in the wild and in laboratory experiments, and reviews, as they say, are mixed at best. Among the genetics community, one will find both proponents and opponents of inbreeding arguing their case with equal vigor.
However, it is interesting to note that historically, many cultures have practiced inbreeding with no evidence of genetic degeneration. Although not alone in the tradition (incestuous marriage was also common under Greek and Roman rule) the most famous were certainly the ancient Egyptians. During the Ptolemy Dynasty, which ruled Egypt during much of its heyday, deliberate inbreeding was common practice among both royalty and commoners alike- examinations of 161 marriage records of commoners in the Ptolemic period reveal that 24% were among siblings. And, astonishingly, every Pharoah from Ptolemy II to Ptolemy XII was the product of a brother/sister marriage! 

What was the result of this?

Cleopatra, who ruled Egypt for twenty-one years during extremely rocky political times, was the product of nearly three hundred years of brother/sister marriages. (323B.C.- 51 B.C.)

Historical records reveal her to have been both extremely beautiful and highly intelligent, with a gift for languages- she was fluent in several.
 Cleopatra also managed to seduce not one, but two, Roman Emperors in succession, producing children by both of them and thereby ensuring the political security of Egypt.

Any suggestion of "inbreeding infertility" is  dispelled by the fact that Cleopatra was herself one of five children born to Ptolemy XII and his wife, and she produced several children herself.

In the face of that evidence, one wonders how so much negativity has come to surround the practice of inbreeding in domestic animals.
Well, several other cultures had less luck with the practice of inbreeding. Most notable was the royalty of Spain and England - where genetic anomalies like hemophaelia and polydactyly abounded, as well as what we would now term developmenta disabilities and mental illness. Infertility ultimately led to the demise of this particuar "pedigree", which was probably ultimately for the best, as these people had the responsibility of running the world's superpowers at the time.

Inbreeding, man has discovered the hard way, is indeed a double-edged sword.

Inbreeding cannot, in of itself, CREATE genetic anomalies. Genes are genes, and unless they mutate, they are simply passed from one generation to the next, in either a recessive or dominant manner. Whether a gene is inherited from a close relative or a perfect stranger does not affect its ability to do its job.
What inbreeding can do is increase homozygosity, (where each gene pair at a given locus contains identical alleles of the gene) and if those alleles are deleterious, a recessive anomaly will present itself. Inbreeding does not so much create genetic problems, then, as reveal them, especially recessive ones which would otherwise remain hidden, and therefore be passed unknowingly to successive generations.
But it is also important to remember that not all recessive traits are deleterious, and desired traits known to be recessive can be locked in just as surely by inbreeding. 

One such trait is the G pairing, a simple color gene allele that causes a dog to silver out at maturity, and common in this breed. (This mutation is not, in of itself, known to be deleterious, unless one wishes to breed dogs who keep the color they are born with, instead of fading out at maturity, which is common in Silks, and often a disappointment to the owner.)
Its recessive allele, g, causes the animal to maintain its early rich color, and probably represents the original gene at that locus, with G  (the so-called "silvering gene") having emerged as an incompletely dominant mutation.

A black dog who does not silver is homozygous (pure) for gg, and does not have the dominant silvering allele in his DNA to pass on. (If he has even a single copy, he would display it, because that is what dominant means.)
So in this case, assuming we wish to breed dogs that do not silver, this "recessive gene" would be desirable.

Many of the traits that we considerable desirable in purebred dogs- indeed, many of which determine breed type- are controlled by recessive genes rather than dominant ones.
Good breeders have long known that the genes that produce proper shoulder layback (which gives Silks that desirable long arched neck and high head carriage) are recessive to those that result in straighter shoulders and a short neck, for example. 
And, the only reliable way to cement these genes into a line and produce animals who display them with consistency, is by inbreeding, or linebreeding (which is slower but somewhat less risky.)
What are the risks?
Well, unlike the gene that prevents a dog from silvering, or those that produce good shoulder layback, many recessive genes cannot be seen with the naked eye, and many are not as desirable.
By linebreeding, many of those genes, if present in the genotype of the original stock, will turn up as unpleasant surprises within the first generation or two. Rigorous selection must be practiced in order to linebreed successfully, and this requires a thoroughly objective eye and a willingness to health screen both the breeding stock and the offspring produced.
If selection is rigorously applied, however, a linebred strain can be consistently healthier, sounder and certainly more attractive than its outbred counterparts.
On the other hand, they may NOT be healthier than the hybrids now being purposefully bred as pets for the pet-owning public, a notion which does not sit well with show breeders as a whole, who have lost a good portion of the pet-owning public to these "upstarts".
Why is this true? The genetic homozygosity needed to consistently produce dogs that fit a breed's standard has recently been shown to unwittingly contribute to autoimmune disease, which presents a unique challenge to responsible breeders who wish to do more than just breed dogs to win. Homozygosity in the DLA genes that regulate immune function have been shown to decrease the body's ability to distinguish between foreign pathogens and its own cells, which is at the root of all autoimmune disease, and linebreeding increases homozygosity across the board. Is there a solution ?
Even the great breeders of the 1930s and 40s such as Onstott and Brackett readily admitted that when faced with problems that could not be solved through selection from within the line, an outcross to an unrelated strain was required.
Should this be deemed necessary (which in the case of the DLA genes it well may be), the best choice is to use a like-to-like breeding of unrelated strains, so as to lose the least in terms of breed type. (I mean, think about it- there is no earthly reason to assume that the genes that control earset are related to the genes that regulate immune function. Although the DLA genes are inherited in chunks known as haplotypes, most of the genes that control physical traits are subject to Mendel's Law of Independent Assortment (which is what makes consistly producing dogs that fit a standard so difficult in the first place.)
In the direst of situations, these outcrosses may need to be another breed entirely, as was done with the Dalmation to reduce high uric acid levels associated with the development of bladder stones.
The REASON the situation has become dire in many breeds is because breeders have not been following the guideines of the early breeders. Instead, in the last 40 years breeders have been linebreeding and inbreeding off nationally and internationally popular sires without regard for preserving the isolated "strains" that existed in the days of Brackett and Onstott, when large breeding kennels maintained closed studs.
These isolated linebred strains constituted "breeds within breeds", and allowed for outcrosses to increase genetic diversity when needed to keep a strain healthy. With the notable exception of Poodles, where isolation between sizes and colors by show breeders has inadvertently resulted in a genetic diversity level unmatched by any other breed, most breeds are in a situation where over 70% of the dogs tested are carrying only two DLA haplotypes, and nearly half are homozygous for one of them.
Whether or not show breeders can let go of their antiquated adoration of "pure blood" inherited from the Victorians in order to breed healthier animals remains to be seen, but it is worth remembering that the early breeders who created the breeds we now strive to preserve had no problem whatsoever adding whatever they needed to the mix to achieve their goals.

Given what is now known, a program of selective linebreeding combined with judicious outcrosses to other breeds when necessary may well prove to be the only available formula for maintaining both breed type (the essential physical and temperamental characteristics that allow us to recognize "breeds" at all) and robust health in the animals we produce.
And those who cannot get their heads around this will ultimate find themselves bloodied by the sharp and ugly underside of linebreeding's double-edged sword.