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#28027 - 09/29/06 06:52 PM Line Breeding, Inbreeding & Outcrossing

This is an article I put together from info from several sources plus comments of my own. I think it pretty much covers most of the angles related to the considerations when using the mentioned breeding methods. I have broken it up into multiple posts as it is long but I think it will help you understand the differences between line breeding, inbreeding & Outcrossing.

Part 1

The Methods and Genetics of Breeding

*Today there is increasing public awareness of the significance of genetic diversity. Greater genetic diversity is considered advantageous in wild animals and plants, in order, for example, to decrease their sensitivity to environmental change. Genetic diversity is also considered advantageous in the total pool of domesticated animals and plants in order to provide source material for improved varieties.

Each individual variety of domesticated plant or animal does not, however, have great genetic diversity. The varieties have been produced by tailoring their genetic makeup to produce a single variety with specific, desired characteristics. The traditional tools to produce individual varieties are crossing and/or breeding in plants and animals. This form of genetic manipulation has been practiced for thousands of years.

The total gene pool (genetic diversity) for chickens is large. This is apparent when one considers all of the various breeds. Each individual breed has been produced, however, by selective breeding which restricts the breed’s gene pool to those genetic elements which produce just that breed. We do not want genes for long legs in a Rosecomb nor do we want genes for short legs in an American Game Fowl. We also do not want genes producing or allowing disease in any breed. The breeding methods used to produce and maintain a chicken breed are the same as those which have been traditionally used to produce and maintain other domestic animals. The following explanation should help you understand some of the more important methods used by careful, successful breeders.

To achieve desired genetic results, all successful breeders, today and in the past, use and have used the system of linebreeding and inbreeding; the results can be both good and bad. A breeder carrying out a serious program is trying, over generations, to keep desired qualities and characteristics of a breed and eliminate or greatly decrease undesirable traits.*

There are three main methods of breeding: outcrossing, linebreeding and inbreeding. Defining the three and understanding the difference between them, is not an easy task for some. There are several other systems of breeding as well. Perhaps we will get into those at another time.

Important definitions to understand:

1. A threadlike linear strand of DNA and associated proteins in the nucleus of eukaryotic cells that carries the genes and functions in the transmission of hereditary information.
2. A circular strand of DNA in bacteria that contains the hereditary information necessary for cell life.

Allele: One member of a pair or series of genes that occupy a specific position on a specific chromosome.

Gamete: A reproductive cell having the haploid number of chromosomes, especially a mature sperm or egg capable of fusing with a gamete of the opposite sex to produce the fertilized egg.

Homozygous: Having the same alleles at a particular gene locus on homologous chromosomes. (Pure in the genetics for a given trait. Example - When a blue-legged bird is carrying genes for only blue legs.)

Heterozygous: Having different alleles at one or more corresponding chromosomal loci. (Impure in the genetics for a given trait. Example - When a blue-legged bird is carrying genes for green legs as well as blue legs. An individual that is heterozygous for a specific trait is carrying unlike genes for that trait, no matter what is expressed in the phenotype.)

Phenotype: Physical appearance. Phenotype is what we "see" (including structure, function, and behavior) in an animal, and 'genotype' is the genetic information the animal carries in its cells and will transmit to offspring. The phenotype along with the known ancestory (pedigree) are our main methods of judging an animal's genetic input.

Genotype: Genetic makeup. Also called genome.

Strain: "A strain is a variety within a variety of animals", obtained after successful linebreedings and inbreedings in a long and serious breeding program.

1. OUTCROSSING: Breeding together unrelated animals or animals that are less related than the average animals of a given breed, variety or strain. Outcrossing or crossbreeding is often defined as "The mating of animals that are members of the same breed, variety or strain but which show no relationship close up in the pedigree." Breeding together animals of different breeds or species is often also referred to as outcrossing or crossbreeding, but we are not really concerned with crossbreeding in that respect so much in this discussion. Sometimes simply crossing to another family line within the same strain (that is somewhat related but not as close) is referred to as outcrossing, and this is the safest way to add new blood while causing the least amount of retrogression in what you have accomplished in linbreeding.

2. LINEBREEDING: Breeding closely related animals, based on multiple pedigree crosses to a single exceptional animal. Linebreeding, is often defined as a breeding method usually directed toward keeping the offspring closely related to some highly admired ancestor, such as sire to daughter, son to mother, brother to sister, half-brother to half-sister, grandfather to granddaughter, grandson to grandmother, unlce to neice, nephew to aunt, cousin to cousin and so on. Technically linebreeding is just a systematic usage of inbreeding.

3. INBREEDING or CLOSE BREEDING: Breeding animals that are more closely related than the average animals of a given breed, variety or strain, such as sire to daughter, mother to son, full-brothers to full-sisters. Although it is a form of linebreeding, 'inbreeding' is often defined as "the mating of closely related individuals, such as sire to daughter, son to mother or full brother to full sister." Inbreeding is defined by some as a much closer cross between the "mating pair" than is linebreeding: son to mother, father to daughter or brother to sister. Some breeders would consider it inbreeding, others linebreeding, when crossing an animal to a half-brother or a half-sister or to one of the cousins, aunts, uncles or grandparents. Technically, there is a continuum from inbreeding to linebreeding just as there is a continuum in degree of relationship between animals of the same breed. As a result, it is sometimes very difficult to give the perfect definition.

INBREEDING ALSO BUT NOT CLOSE: Grandfather to granddaughter, grandson to grandmother, uncle to neice, aunt to nephew, cousin to cousin, half-brother to half-sister.

VERY CLOSE INBREEDING: Breeding daughters back to their sire for successive generations, sons back to their mother for successive generations, or full-brothers to full-sisters out of full-brothers and full-sisters for successive generations.

Outcrossing -
By mating stock that are unrelated, the chance of intensifying undesirable traits is minimized. Unfortunately, due to the heterozygous, or dissimilar, genetic nature of a group of unrelated stock, the chance of intensifying desirable traits is likewise deminished. In theory, stock that are the product of outcrossing programs (Also called Crossbreeding) may be more heterozygous than inbred stock. Crossbreds might be outstanding performers, but as breeding stock, they will not pass on their desirable characteristics consistently. Often the most consistent results in an outcrossing program are obtained when the pure strain individual used in the cross is, himself, a strongly linebred individual.

"So-called successful nicking (Offspring greater than dam and sire) is due, genetically speaking, to the fact that the right combination of genes for good character are contributed by each parent, although each of the parents within itself may be lacking in certain genes necessary for excellence." "In other words, the animals nicked well because their respective combinations of good genes were such as to complement each other."
However, all outstanding animals arising from this method of breeding (Outcrossing) should be carefully scrutinized from a breeding standpoint, because, with their heterozygous origins, it is unlikely that they will breed true.

*Outcrossing is a less blood-related connection, that is, the sire and dam are not related or at least not closely related. Sometimes after many generations of linebreeding and inbreeding, it is time to outcross. This also represents a danger for a breeder without experience especially if the outcross is to be made to a totally unrelated strain.

"After an outcross has been made, a breeder should then breed right back into the original strain. This is the only safe procedure after the desired trait has been obtained and the purpose of the outcross has been achieved." "A strain is a variety within a variety of animals", obtained after successful linebreedings and inbreedings in a long and serious breeding program.

It is almost impossible to explain when it is time to outcross. After too many generations of linebreeding, there is sometimes a risk of producing non-desirable traits. Not all breeders agree that these risks occur, but many have experienced problems which they have attributed to linebreeding.

When a breeder outcrosses, he can lose the type he has long worked to produce. He needs to introduce new genetic material only if the animal used can bring something needed into his program and if he has linebred and/or inbred already several times. The breeder needs to know perfectly the ancestors of this "external" animal and what those ancestors can add to the line he has produced. In many cases, the level of danger inherent in inbreeding, linebreeding, and outcrossing can be equal when these breeding methods are practiced by a careless or inexperienced breeder.

The breeder must go back, after an outcross, into his established line, to perpetuate the "good" brought into that line by the outcross. Of course it is also time to use an outcross when a breeder has not produced what he has wished or when he can have access to a superior animal or a very rare and proven pedigree which is not yet in his program. This system can bring qualities he needs, or correct a fault he has not been able to eliminate. Some people say that outcrossing can bring vigor, but linebreeding, when very carefully planned, should have the same result because vigor is a desirable trait which a breeder should linebreed for. The outcross can also bring undesirable traits. The breeder will have to hold the desirable qualities he has obtained with the outcross by returning to linebreeding with the progeny and should do the same to eliminate the bad traits brought by the outcross.

The general public believes that inbreeding produces lack of vigor, poor fertility and bad temperament and forgets that the same problems can arise with all other systems of breeding. It is the breeder’s responsibility, in any breeding program, to select for desirable characteristics and strenuously guard against undesirable ones. Of course, close inbreeding is full of dangers, but it is a great tool when used by a careful breeder who knows the qualities and the faults of his stock and their ancestors. He must be able to recognize sound conformation and temperament and it helps to be knowledgeable in the area of the genetics pertaining to the stock he is breeding.*

#28028 - 09/29/06 06:54 PM Re: Line Breeding, Inbreeding & Outcrossing

Part 2

Linebreeding -
Linebreeding is built on the princple of "breeding like to like to get like." It affords the serious breeders the opportunity to set a type in the shortest amount of time.

(The late Hank Wiescamp of Alamosa, Colorado, was in the opinion of some, the undisputed king of horse linebreeders, Utilizing 'Skipper W. AQHA' as the cornerstone of his program, Weiscamp linebred a "family" of Paint, Quarter Horses and Appaloosas that were so easily distinguishable by coloring and type that they are more often described as " Wiescamp horses" than as members of any single breed.

The greatest danger in a linebreeding program is that it intestenifies all of the genes--good and bad. In other words, if you have a line of inherently gifted racehorses, breeding like to like--speed to speed--should result in an ever-faster line of horses. If, at the same time, this line of horses is consistently bad-tempered, breeding like to like should result in an intensified line of poor-dispositioned horses.

Inbreeding -
Inbreeding is an intensified form of linebreeding, with the sole difference being in the genetic closeness of the animals being bred to each other. Among the advantages of inbreeding, are that it affords the surest and quickest method of fixing and perpetuating a desirable characteristic or group of characteristics. It tends to create lines or strains of animals that are uniform in type, and it keeps the closest possible relationship to a desirable ancestor. The disadvantages of inbreeding, are that it sometimes increases the proportion of undesirable breeding stock, as genetic abnormalities may appear with increased frequency. Inbreeding, requires skill in making planned matings and rigid selection, thus being most successful when applied by 'master breeders.' "

*The purpose of both linebreeding and inbreeding is to bring improvement more rapidly and narrow the pedigree to a few closely related lines of descent. "This purifies the pedigree rapidly and enables a breeder to control, to some degrees, all characteristics". Both linebreeding and inbreeding reduce genetic variability, often with the aim of eliminating genes for undesirable recessive characteristics. It is easier to predict the results of breeding by linebreeding than to expect results from a breeding program that has no genetic basis. If the selection of the ancestors has been good, no or very few surprises should be expected.

Judicious linebreedings have long yielded, in many breeds of domestic animals, important and real improvements. The quickest and most certain way to produce superior animals is by careful linebreeding, especially with a breed having little or no genetic susceptibility to disease.

However, there is a great danger in this system of breeding if the breeder considers only the pedigrees, without considering the physical, functional, and behavioral characteristics of the animals being bred and of their ancestors. Inevitably, the results are offspring with notable faults. A pedigree is a guarantee only of bloodlines and not of the qualities within those bloodlines. Unfortunately many beginners will consider only pedigrees, especially if they contain winners. They will breed without considering also the respective faults and qualities of the animals they want to mate. Close inbreeding will concentrate not only the desirable qualities, but also the faults. Fortunately truly serious breeders will study very carefully the properties the pedigrees represent. That is they will study the family of the animals they want to mate (the qualities, the faults, the temperament, the results also in show, racing, etc) before linebreeding, to fix a type in their breeding program. That type will be the closest possible to the breed standard with robust health and desirable temperament.

Linebreeding provides for early recognition of undesirable characteristics. A fault which is not apparent in the ancestors in the pedigree can come out in a first or second generation (so can deisrable traits that are not apparent), because of the enhanced probability of the occurrence of two recessive genes for the same trait. However, without the closeness of line breeding, the fault will probably appear much later and so, will take much more time to be eliminated. We are fortunate that chicks are born in bunches of non-identical "twins" so that the probability of the fault appearing in all siblings is not too great.

Inbreeding is linebreeding in its limits. The advantages and disadvantages of linebreeding reach their highest degree with inbreeding. This system of breeding is to maximize the probability that all the qualities of the sire, in an example where the sire breeds his daughter, are retained in the offspring. If inbreeding is practiced, it is absolutely essential to study perfectly the ancestors of the sire in order to know their qualities and faults. Those faults must not appear in the daughter or in her pedigree because of the enhanced probability that the father x daughter offspring will inherit the undesirable genes of the sire. It is imperative to choose a sire who has the qualities absolutely opposite to the daughter’s faults and the daughter should have none of the faults of her sire, who in this example, is the sire of her offspring. Not only the conformation, but also the temperament, disease resistance and reproductive qualities of the two animals being mated by linebreeding or inbreeding must be taken very seriously into consideration with the objective of securing and preserving only the desirable qualities.

When superior animals are used, inbreeding is the most powerful way to perpetuate their qualities and to influence the future of a breeding program. "Inbreeding is not so much a matter of originating excellence as of holding and making the greatest use of it when it appears". Inbreeding can be a disaster if practiced by a breeder beginning with the wrong stock, or not knowing perfectly the breed, or not knowing what is a very good individual. In this case, anyway, he or she is not ready to be a serious breeder.

Good breeders linebreed or inbreed only superior individuals. A properly linebred or inbred animal is much more capable of reproducing its type than an animal which has outcross breeding.

"It is known universally that any characteristics can be bred up or down, strengthened or weakened by inbreeding." Severe selection is the key to successful linebreeding or inbreeding; the good trait in one parent should be stronger than the off-setting undesirable trait in the other parent.

When a breeder regularly produces offspring with too much variance in type, he needs to consider linebreeding. He will have the possibility of seeing the traits (dominant and recessive) in his stock. It is a risk, of course, but it is the only way for him to progress and try to produce homogeneous offspring. Linebreeding and inbreeding are not ways to bring new characteristics, but are proven ways to consolidate the best traits.*

Pedigrees and Breeding Schemes
Breeders cannot change Mendelian genetics, nor the number of genes involved in traits, nor their linkage relationships. They cannot change the physiological interactions of gene products, but they can hope through selective mating to produce gene combinations that constistently result in high quality stock.

Broadly speaking the systems of mating that a breeder may choose are:

1. Mating like to like (based on pedigree likeness or individual likeness, such as performance success, disposition or body shape).
2. Random mating (no selection)
3. Mating unlike to unlike (based on outcrossed pedigrees or individual extremes, such as tall with short, or lanky with compact, or hot temperament with mellow).

Successful examples of all these schemes could be cited for any breed. Every breeder needs to understand the genetics principle underlying these situations and then decide which is most appropriate for each breeding pair.

Assortive Mating
Positive Assortative Mating - Mating like to like on the basis of their perceived similarities, without regard to pedigree. This technique could combine animals with similar genes or animals with different genes affecting a similar phenotype. In any case, the goal is to produce an animal that closely resembles the parents.

Negative Assortative Mating - Mating together animals of unlike phenotypes. The goal of this process is to breed offspring that are not as extreme as either parent.

Relatedness and "percentage of blood"
Offspring resemble their parents to varying degrees, but the proportional genetics contribution of each is constant: half the genes of an offspring come from the mother and half from the father - A relatedness or inbreeding coefficient of 50% or 0.5 assigned to the parent-offspring relationship.

Full siblings on average share 50% of their genes. For every locus, assuming the parents are heterozygous for different alleles, the offspring have four possible allelic combinations: 25% of the time they will have received the same alleles from their dam; 25% they will receive the same alleles from their sire; 25% of the time they will have received no alleles in common from sire or dam; 25% of the time they will have received the same alleles from both sire and dam. The random assortment of chromosome pairs during gamete formation means that we cannot predict the exact proportion of genes that any two full siblings have in common; we can only provide an average value for all genes of full siblings as a group.

Half-siblings on average share 25% of their genes, and first cousin share about 12.5%. Relatedness decreases by half with each succeeding generation. More complicated relationships can also be calculated.

The genome of any animal is always a composite of contributions from the entire pedigree. The fraction of genes in an offspring's genome attributed to a particular ancestor (appearing only once in the pedigree) in distant generations is small. Any single gene in a fifth generation ancestor has only about a 3% chance of pass through all the meiotic partitions and arriving in the genome of the great-great-great-grandchild. However the chances become greater when a particular ancestor appears multiple times in the pedigree.

Toward Homozygosity
The theoretical purpose of inbreeding (or linebreeding) is to produce stock of consistent excellence through enhanced relationship to admired individual ancestors. Inbreeding not only increases the proportion of genes that trace to a given ancestor, it also increases the likelihood that the genes will be homozygous. The premise underlying the positive attitude toward inbreeding is that homozygosity is desirable, but these genes may not have been homozygous in the admired ancestor. Key components for working with inbred pedigrees to avoid producing individuals homozygous for an undesired trait include accurate tests to identify trait carriers and knowledge of basic genetics.

#28029 - 09/29/06 07:55 PM Re: Line Breeding, Inbreeding & Outcrossing

One of the main things I want you to get from this is that line breeding is not always as bad as many make it out to be. You can actually improve the vigor in a line by line breeding, by selecting for the absolute healthiest and vigorous stock year after year and culling every bird that shows any weakness at all in disease resistance, reproductive quality or serious physical fault. This does not mean you should try to line breed them closer and closer and closer year after year, but you can line breed fairly close to get them how you want them, and then spread them out into different family lines while they are still vigorous to maintain that vigor.

On the other hand you can sometimes breed weakness into your strain by outcrossing. For example if your strain has a high resistance to a particular disease and then you outcross to a bird that has little or no resistance to that disease then you are breeding the genetics for weakness to that disease into your strain. You may or may not see that weakness in the f1 but you probably will in future generations as the recessive genes combine in certain of the offspring. Now if you were to decide to start inbreeding these birds that contain that weakness and if by chance you selected down to where all the birds you were breeding from carried that weakness in a pure form (you kept them alive by medication or they just had not been exposed to the disease as of yet) then this would be where inbreeding would become a bad thing. Not because the birds are closely related but because you have narrowed the genetic pool down so small that there is no variation to select against the detrimental trait.

An outstanding breeder can get away with a lot more line breeding (or inbreeding) than a negligent or less observant breeder because he/she studies his fowl very close on every possible trait that can be assessed, and he studies the pedigree to take into consideration any weakness that is likely to be carried but not able to be assessed by observation. He then mates the birds in a way to produce the most consistent uniformity in phenotype while also striving to avoid mating together birds that are likely to carry that same weakness.

As just one of many possible examples: the breeder may maintain 3 lines of closely related families of the same strain, of which all contain 7/8 the blood of one outstanding bird that has minimal serious faults. Although all these lines are closely related they all contain 1/8 different blood. This 1/8 different blood is often enough to keep the line from running into inbreeding depression. If a weakness does arise the breeder can outcross between the different lines to maintain enough variation to avoid inbreeding depression.

Yes you will get some weaklings when inbreeding for any length of time, but you will also get weakness from outcrosses too, though they may not show up in the beginning. Sooner or later inbreeding to some extent is going to have to be practiced to maintain any consistency and uniformity in the line. All pure breds were made by inbreeding to set the desired traits in the strain.

I guess this is mainly meant for those that want their chickens to turn out about the same quality or improve every year, not just in phenotype but as an all around well bred high quality fowl. The best breeders usually specialize in just a few strains and they observe them closely down to the fine details of every aspect and the pedigree.

#28030 - 09/29/06 08:02 PM Re: Line Breeding, Inbreeding & Outcrossing

I think someone in another thread asked which was closer, parent to offspring or brother x sister.

Inbreeding comparisons

Situation # 1: An unrelated male and female are bred together, then the daughters bred back to the father for successive generations. Percents are the 'in-breeding coefficient'. (Remeber that in order to obtain much uniformity by this form of line breeding it is important that the parent being bred back to be homozygous for a good number of the traits he possesses)

1st daughter = 0%
2nd daughter = 25%
3rd daughter = 37.5%
4th daughter = 43.7%
5th daughter = 46.9%
6th daughter = 48.4%
7th daughter = 49.2%
8th daughter = 49.6%

Situation # 2: From an unrelated pair, brothers and sisters are mated together generation after generation.

1st generation = 0%
2nd generation = 25%
3rd generation = 37.5%
4th generation = 50.0%
5th generation = 59.4%
6th generation = 67.2%
7th generation = 73.4%
8th generation = 78.5%

Conclusion: Early on there is not much difference between parent x offspring matings and brother x sister matings, but after a few generations the brother x sister matings become more inbred more quickly.


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