The Genetic and Phenotypic Consequences of Introducing
Additional Pedigreed Siamese in Generations 6-8:
Why Pedigree Depth Matters?
prepared by Katia Kingston 2026
Introduction
An established breed is the product of generations of selection. Breeders select animals that consistently express the desired characteristics of the breed. Over time, these characteristics become associated with the genetic background of the population. Breed identity is therefore much more than a single color, a single gene, or a small collection of DNA markers. It results from complex interactions among many genes and genetic variants that have been selected and maintained within the population over generations.
When another breed is introduced through an outcross that genetic history changes. This remains true even when the outcross occurs six, seven, or eight generations back. An outcross introduces an entire genetic background from another population. Some of that genetic material may be beneficial, some neutral, and some undesirable or regressive. Some effects may be immediately visible, while others may not become apparent until later generations—when the genetic material has already been passed on and recombined.
The cited scientific literature demonstrates several important principles:
- Outcrossing changes the genetic composition of a population.
- Genetic material from a donor population will remain in the recipient population and may have negative effects.
- The phenotypic consequences of outbreeding can vary among traits and generations and will dilute homozygosity.
- Backcrossing can reduce donor ancestry, and cannot guarantee restoration of the original population.
- For an established breed developed through generations of selective breeding, maintaining an eight-generation pedigree requirement represents a prudent population-preservation measure.
An Outcross Changes the Population
When an established breed is crossed with another breed, the offspring receives genetic material from both populations. The resulting animal is therefore genetically different from the animals that previously composed the established population. An outcross does not introduce only the specific characteristic that motivated the cross. It introduces genetic material from the donor population as a whole. One of the cited studies examined the fate of alleles following outcrossing. It found that lethal alleles were less likely to be introduced through breeding within the population because natural selection had already reduced their frequency. Outcrossing, however, allowed slightly detrimental and neutral alleles to be introduced more frequently. The significance is clear: an outcross cannot selectively transfer only the desired genetic material. Therefore, the genetic consequences of an outcross can extend beyond the particular trait that motivated the breeding decision.
New Genetic Material Can Affect Phenotype
The introduction of new genetic material can also affect phenotype. One of the studies evaluated fitness, morphological, physiological, and other phenotypic characteristics. The researchers found that the effects of outbreeding were not uniform. Some traits benefited from outcrossing, while others experienced negative effects. Importantly, the response depended on both the type of trait and the generation being examined. This means that the consequences of combining populations cannot necessarily be predicted from the original purpose of the cross. A breeder may make an outcross to introduce one desirable characteristic, but other genetic effects may accompany that cross. Some may not become apparent until later generations. The absence of an immediately visible negative phenotype does not prove that the outcross had no genetic consequences. The evidence becomes especially relevant when examining cat breeds.
The researchers found that population structure in cat breeds generally reflected their ancestry, although some breeds showed weaker population definition. Most importantly, they identified 16 breeds that could not be resolved as independent populations. The researchers attributed this finding to outcrossing between established breeds and recently developed breeds with common ancestry. This demonstrates that outcrossing can affect the genetic distinction between established breed populations. This raises an important question for the Siamese breed: Do we want to introduce genetic material from other breeds into the Siamese population at generations six through eight?
The issue is not simply whether an individual cat looks like a Siamese. The issue is whether the genetic continuity of an established Siamese population should be protected.
Backcrossing Requires Generations
After an initial outcross, approximately half of the nuclear genetic material is expected to come from the donor population. With repeated backcrossing to the established breed, the expected proportion of donor ancestry decreases. However, there are no guarantees of “improvement.” This is exactly why generations matter. Five generations do not produce the same expected reduction in donor ancestry as eight generations. The additional generations provide more opportunities for:
- recombination of genetic material;
- selection;
- reduction of donor ancestry;
- evaluation of offspring; and
- breeding back into the established population.
Therefore, the difference between five and eight generations is not simply a difference on a registration document. It represents three additional generations of genetic and population history.
Returning to the Original Breed Is Not a Simple Process
Once another population has been introduced, it may be tempting to assume that repeated backcrossing to the established breed will eventually recreate the original population. The process is not that simple. Backcrossing reduces the expected proportion of donor ancestry, but it does not guarantee that the original genetic arrangement will be reconstructed. Genes are inherited and recombined. They are not returned to their original positions and combinations.
A breeder may successfully select for the desired appearance while portions of the donor genome remain present but are not readily observable. This creates a fundamental difference between phenotypic resemblance and genetic restoration.
An animal can resemble the established breed without possessing the same genetic composition as an animal whose ancestry has remained entirely within that breed for many generations. An AOV is an example of this distinction. Therefore, the claim that an animal has “returned” to the original breed cannot be established solely by appearance. A phenotype can be selected. Genetic history cannot simply be erased.
Breed Characteristics Are Complex
Animal populations contain large collections of genetic variants that affect phenotype. Traits such as growth, development, metabolism, reproduction, behavior, and physical characteristics are influenced by genetic variation. Breeding programs have historically enriched particular combinations of mutations and genetic variants because they produce desirable phenotypes. A breed’s phenotype is therefore the product of a complex genetic background. Changing that genetic background can change the way individual genes are expressed and how combinations of genes interact. This is why breed identity cannot be reduced to a single gene, a single color, or a limited number of DNA markers. The genetic background matters.
Eight Generations Provide More Evidence Than Five
This brings the issue directly to the proposed reduction from eight generations to five. An eight-generation pedigree documents three additional generations of ancestry compared with a five-generation pedigree. Those additional generations provide more opportunities for:
- selection for the established breed phenotype;
- reduction of donor ancestry;
- recombination of genetic material;
- identification of undesirable characteristics;
- evaluation of offspring; and
- continued breeding within the established population.
The difference is therefore not simply three additional lines on a registration document. It represents three additional generations of population history. Reducing the requirement removes part of that historical evidence. Once animals carrying recent donor ancestry are bred within the population, their genes can be passed to subsequent generations and recombined with the established breed’s genetic background. Over time, genetic material that was once foreign to the population can become part of the population itself. The question then becomes: What happens when the registration standard repeatedly permits animals with less documented breed ancestry to enter an established population? The concern is that genetic material from another population can gradually become incorporated into the established breed.
Prevention Is More Reliable Than Reversal
The scientific evidence supports a cautious approach. Once genetic material from another population has entered an established breed, removing it completely can be extremely difficult. Selection and backcrossing can reduce donor ancestry. However, genetic recombination means that the original genomic arrangement cannot simply be recreated. This is the critical distinction: Reducing donor ancestry is not the same as restoring the original population. The fundamental breeding principle is that it is easier to protect an established population from unnecessary introgression than it is to reconstruct its original genetic and phenotypic characteristics after introgression has occurred.
A pedigree requirement is therefore a preventative measure. It provides documented evidence of ancestry before an animal is incorporated into the established population.
What Is at Stake?
The scientifically defensible concern is that reducing the pedigree requirement lowers the amount of documented ancestry required to establish genetic continuity with the existing breed population. That creates less certainty about the animal’s genetic history. For a developing population, this may be acceptable. For a well-established breed such as the Siamese, whose identity depends upon generations of selective breeding, the objective is different. The objective is preservation of an existing population rather than intentional alteration of that population. When preservation is the goal, maintaining stronger evidence of ancestry is a rational safeguard. The issue is therefore not whether outcrossing can produce a desirable individual. It can. The issue is whether reducing the pedigree requirement creates a greater opportunity for recent donor ancestry with undesirable traits to enter and become established within an existing breed population.
Conclusion
Outcrossing is a powerful and irreversible genetic intervention. It changes the genetic composition of the recipient population. An outcross introduces genetic material from an entire donor population, not simply the desired trait. Donor alleles can persist for multiple generations, and genetic material can be recombined and distributed through subsequent generations. Phenotypic effects may not be immediately apparent. Backcrossing can reduce donor ancestry, and it cannot guarantee restoration of the original genetic arrangement or phenotype. This is why pedigree depth matters.
An eight-generation pedigree provides three more generations of documented ancestry than a five-generation pedigree. Those additional generations provide additional time for recombination, selection, evaluation, and reduction of donor ancestry. The difference between five and eight generations is therefore not merely administrative. It represents a difference in documented genetic and population history. Once genetic material from another breed becomes established within a population, completely returning that population to its previous genetic and phenotypic state is extraordinarily difficult and can become impossible. For an established breed whose identity has been developed through generations of selective breeding, the goal is preservation of that population.
Maintaining the eight-generation pedigree requirement therefore represents a prudent population-preservation measure and a safeguard against unnecessary introgression.
Works Cited
Adavoudi, Roya, and Małgorzata Pilot. “Consequences of Hybridization in Mammals: A Systematic Review.” Genes, vol. 13, no. 1, 2022, article 50. https://doi.org/10.3390/genes13010050
Andersson, Leif. “Genetic Dissection of Phenotypic Diversity in Farm Animals.” Nature Reviews Genetics, vol. 2, 2001, pp. 130–138. https://doi.org/10.1038/35052563
Kaelin, Christopher B., et al. “Ancestry Dynamics and Trait Selection in a Designer Cat Breed.” Current Biology, vol. 34, no. 7, 2024, pp. 1506–1518.e7. https://doi.org/10.1016/j.cub.2024.02.075
Menotti-Raymond, Marilyn, et al. “Patterns of Molecular Genetic Variation among Cat Breeds.” Genomics, vol. 91, no. 1, 2008, pp. 1–11. https://doi.org/10.1016/j.ygeno.2007.08.008
Twyford, A. D., and R. A. Ennos. “Next-Generation Hybridization and Introgression.” Heredity, vol. 108, no. 3, 2012, pp. 179–189. https://doi.org/10.1038/hdy.2011.68
Whitlock, Raj, et al. “A Systematic Review of Phenotypic Responses to Between-Population Outbreeding.” Environmental Evidence, vol. 2, 2013, article 13. https://doi.org/10.1186/2047-2382-2-13
Windig, J. J., and H. P. Doekes. “Limits to Genetic Rescue by Outcross in Pedigree Dogs.” Journal of Animal Breeding and Genetics, vol. 135, no. 3, 2018, pp. 238–248. https://doi.org/10.1111/jbg.12330
Zayas, Gabriel A., et al. “Maintaining Breed Integrity: Successful Introgression of the SLICK1 Allele into the Holstein Breed.” Journal of Heredity, vol. 116, no. 3, 2025, pp. 216–224. https://doi.org/10.1093/jhered/esae057