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For some breeders, breeding is a game of chance or luck, in that they put two animals together and hope for the best without a long range plan or goal in mind. For instance, if you want to add ear size and lynx tips or lighter coat color, there are two ways to go about it. (1) is to research lines or pedigrees to see how dominant that particular trait is and the other is to breed to a cat that has the appearance of that trait. The first way is breeding by Genotype, the second by Phenotype.
When setting up your breeding program, please realize it is very hard to change, fix or create a trait in a single generation. The more information you have on the ancestors the easier it is to work your program - remember that all breeds within a species are genetically the same, so what at first looks like an out cross may still be closely related.
Tens of thousands of genes go into the makeup of one cat. Genes are inherited in pairs - one pair from the mother, one pair from the father - if the pairs from both parents are the same they are called homozygous, if different they are called heterozygous. Genes that make a breed, breed true are there for homozygous. In each breed exists a considerable amount of homozygous non-variable pairs, these control things like size and color differences in a breed.
Each breed is developed by close breeding and inbreeding among a small group of founding ancestors. This process creates the breeds characteristics and makes the cats in the particular breed, breed true. Breeding closely related individuals increases the chance that identical genes from a common ancestor will be passed down from both the sire and the dams side of the pedigree. Again if you wanted to set the look of larger ears and lynx tips you would research pedigrees looking for cats on both sides carrying these traits.
Line breeding and Inbreeding are both examples of breeding by Genotype. While this creates greater homozygosity in the offspring by fixing desirable traits, it also allows for a greater expression of recessive genes that are not expressed in the heterozygous state. For instance if you have a recessive trait in a heterozygous breeding that is rare in the population, it can be passed from a common ancestor on both the sire and dams side creating a homozygous recessive in the offspring. Inbreeding does not create undesirable genes it simply increases those that are present in the heterozygous state.
There are many views on inbreeding as a measurable number (called the inbreeding coefficient or Wrights coefficient) that goes up when there is a common ancestor between the sire and the dams side of the pedigree. Some people consider inbreeding to close when it is brother/sister or father/daughter. a common ancestor even in the eighth generation will considerably increase the amount of inbreeding in a pedigree. The inbreeding coefficient is an estimation of all the variable genes that are homozygous due to inheritance from common ancestors, or the chance that only one gene pair is homozygous due to inheritance from a common ancestor.
For a calculated coefficient of a pedigree to be accurate it must be tabulated from a number of generations. This is because inbreeding in the 5th and later generations (background inbreeding) often has a profound effect on the offspring today.
In studies conducted the difference in inbreeding coefficients based on four versus eight generations is considerable. For instance a 4 generation pedigree with 28 unique ancestors out of 30 positions may show a low inbreeding coefficient, while 8 generations of the same pedigree with 212 unique ancestors our of 510 possibilities would have a considerably higher coefficient. Knowledge of the depth of the pedigree is always good in pairing two cats. What seems like an outcross in the first couple of generations may appear as a line bred concentration of genes for influential ancestors in extended generations.
Take first a cousin example of mating in four generations (28 out of 30 different ancestors) the coefficient is 7.81 percent, using the 5th generation it would be 9.86, the 6th generation is 16.64 percent, 7th is20.83 percent and 8th is 23.22 percent - which is almost the same as parent offspring, as you can see background inbreeding has far more influence than the appeared 1st cousin in the 4 generation pedigree.
Rare or newer breeds will have higher coefficient numbers than older established breeds simply based on the number of available relatively unrelated breeding stock. While 10%based on an 8 generation pedigree may be moderate inbreeding for and established breed with a low average inbreeding coefficient, it may be considered an out cross for a newer breed with a relatively higher inbreeding coefficient.
The knowledge of the degree of inbreeding only helps if you know what genes are being concentrated and passed from a common ancestor. A parent passes (sire and dam) 50%, grandparent 25%, great grandparent 12.5% etc. For every time an ancestor appears in the pedigree its percentage of genes can be added up and its percentage of blood estimated.
The relationship coefficient will vary from the percentage of blood due to differences in the inbreeding coefficients and the pedigree as a whole - example if an ancestor has a higher inbreeding coefficient and therefore greater homozygosity, the pair will always represent the ancestor.
If the ancestor is relatively out crossed, therefore has greater heterozygosity, then a single gene from the pair will only represent 50% of the types of genes available to its offspring. Such an animals descendants will have fewer total genes that will be the same as the ancestor. The relationship coefficient will reflect this by being lower than the percentage by blood.
In line breeding it is better for line bred ancestors to appear
on both sire and dams side of the pedigree - that way their genes have
a better chance of pairing back up in the resultant offspring. To
reproduce or concentrate the genes of and ancestor, you should use a number
of its offspring in the pedigree that share the ancestors best traits.
If an ancestor appears a number of times in a pedigree, but always behind
the same offspring, you are only concentrating the approximate 50% of genes
passed from the ancestor to the single offspring. You are line breeding
the offspring, not
the ancestor. In many breeds, an influential individual may not
appear until later generations, but then will appear so many times that
it contributes a large population of genes to the pedigree.
Many breeders breed by appearance (phenotype) of an animal and not on the pedigree or relatedness of the prospective parents. These are called Assortative matings. There are positive assortative matings (like to like) and negative assortative matings (like to unlike) for individual traits. We use positive assortative matings when we establish or fix traits and negative assortative matings when we wish correct or bring in traits the breeding stock may lack.
Each cat/kitten with the same pedigree will not necessarily look or breed alike - therefore breedings should not be planned solely on pedigree or appearance alone - instead a combination of appearance and ancestry. If you are trying to set a trait and it appears on both the parents and the line bred ancestors of two related cats your chances are better for attaining your goal. If a line breeding produces a magnificent kitten whose qualities are not present in the ancestors, then the cat may have a wonderful show carrier, but may not breed true. Therefore a careful selection of mates is important, and a careful selection of kittens from the resultant litter is also important to attain your goals.
Breedings with inbreeding coefficients lower than average for the breed are produced by out crossing and tend to increase heterozygosity (matching pairs of unrelated genes from different ancestors). You can generally expect non-uniformity in these litters, the exception being if the parents are so dissimilar that they create a uniformity of heterozygosity. This is what usually happens in a mismating between two breeds. The resultant litter tends to be uniform exhibiting ôhalf-way pointsö between the dissimilar traits of the parents. While these offspring may look alike they rarely breed true due to the mix of dissimilar genes. This can be a useful tool to bring in traits you do not have in your breeding stock or to correct faults.
Inbreeding will quickly identify the good and bad recessive genes the parents share in the offspring. Unless you have prior knowledge of what the kittens of the line bred ancestors were like, you may be exposing your kittens and therefore kitten buyers to extraordinary risk of genetic defects.
If your breeding program is stagnating after several out crosses and there is no direction to go it is because you have an abundance of heterozygosity - little uniformity and diminished response to selection.
If you run into trouble with a line breeding program, you can always out cross and in one generation, which will bring in new genes and allow you to immediately change direction toward more specific goals.
Dont set too many goals in each generation - genetically complex or dominant traits should be addressed early in a long range breeding plan. as they take several generations to fix. Traits with major dominant genes become set more slowly as the heterozygous animal will not be easy to tell from the homozygous - dominant animal. Desirable recessive traits can be set in one generation as the animal that shows the characteristics are homozygous for the recessive genes. Cats that breed true for numerous matins and generations should be preferentially selected for breeding stock. This is due to homozygosity of dominant and recessive genes.
This material was take from an article by DR. Bell director of the Clinical Veterinary Genetics Course of the Tufts University School of Veterinary Medicine. He teaches courses in genetics, practices small animal medicine and breeds Gordon Setters.
I am presently using the Cattery Standard Pedigree Program with Mega
Pad - it has a genetics program and will calculate the inbreeding coefficients
- it will also track genetic traits. I am compiling a data bank and
have already entered many of the Pixiebobs. I am keeping track
of color, pattern, feet (poly or straight), and tail lengths,
as well as heritage. This is a data bank that is for all of us and in the
future may be beneficial to any of us looking for links or filling in blank
spaces in our pedigrees. I invite all who wish to participate to
contact me.
