Introduction from Vlastar Lux Cattery
At Vlastar Lux Cattery, our philosophy is rooted in responsible, transparent, and science‑based breeding. We believe that every breeder should clearly understand the genetic tools that shape a healthy, stable, and predictable breed. One of these tools — often discussed, sometimes misunderstood — is inbreeding.
The article “Inbreeding — Pro et Contra” by M.M. Aslanyan and A. Spirin is a classic scientific overview that remains highly relevant for modern feline breeding. It explains, in accessible language, how inbreeding functions at the genetic level, why it has both advantages and risks, and how professional breeders use it deliberately — not accidentally — to strengthen breed type, reveal hidden recessive genes, and build consistent, high‑quality breeding lines.
At Vlastar Lux Cattery, we support the principle that genetic knowledge is the foundation of ethical breeding. Inbreeding is not a shortcut and not a method for mass production — it is a precise and carefully controlled tool used only when it serves the long‑term health and development of the breed. When applied with discipline, knowledge, and rigorous selection standards, it helps breeders:
- consolidate desirable traits,
- remove harmful recessive mutations,
- maintain genetic consistency, and
- create strong, predictable bloodlines.
This article provides an excellent scientific context for understanding why responsible breeders sometimes use planned inbreeding — and why it must always be accompanied by strict selection, health monitoring, and a deep understanding of feline genetics.
We encourage our readers, clients, and fellow breeders to approach breeding with respect, knowledge, and long‑term vision. Our cattery remains committed to the highest standards of genetic integrity, health, and breed excellence.
Vlastar Lux Cattery —
Where science, beauty, and responsibility come together.
“INBREEDING – PRO ET CONTRA” – M. Aslanyan, Professor of the Department of Genetics and Breeding, Doctor of Biological Sciences;A. Spirin, Professor of the Department of Molecular Biology, Doctor of Biological Sciences, Full Member of the Russian Academy of Sciences© Magazine “Drug” 1996 – 6
This article is addressed to cat enthusiasts and cattery owners, as well as to the chairpersons of clubs and associations whose main goal is to strengthen and improve breed qualities of their animals through purebred breeding and selection. Let us emphasize immediately that because selection involves strict culling, this goal is in conflict with commercial cat breeding, where the main goal is profit.
Inbreeding as a Component of Purebred Breeding
In the course of their work, a breeder‑selector must decide which females should be mated with which of the selected males so that their offspring combine specific traits and acquire improved characteristics. This selective activity is called pair selection for mating. Pair selection must be carried out systematically and consistently, with the aim of changing the genetic structure of the animal population of a given cattery in a desirable direction or preserving and fixing in the offspring certain combinations of valuable traits.
Sometimes, to create new breeds or introduce completely new characteristics not previously inherent to a breed, interbreed or even interspecies crosses are used. However, the main direction of the breeder’s work is usually purebred breeding—mating individuals of the same breed.
Purebred breeding is divided into two types:
- Outbreeding – a system of non‑related matings within a breed.
- Inbreeding – a system of matings between individuals with a close degree of kinship: brother–sister, father–daughter, mother–son, cousins, etc.
In general, inbreeding implies that the mating individuals—the future father and mother—have common ancestors or at least one shared ancestor.
Although for specialists the importance and necessity of inbreeding in purebred breeding are obvious, among amateurs there is a multitude of myths and prejudices, both against inbreeding itself and against the use of inbred breeders in reproduction. (We will say in advance that the latter is an especially serious mistake, since inbred, carefully selected breeding animals are usually prepotent—their offspring tend to “take after the father”—and they often produce exceptionally strong litters.)
What Is the Biological (Genetic) Essence of Inbreeding?
All modern domestic animal breeds, including cats, are heterozygous for many genes. This means the following:
A zygote—an egg fertilized by a sperm—receives from both the father and the mother one complete set of genes, and therefore has a double set. If both genes responsible for a given trait—one from the father and one from the mother—are identical, this state is called homozygous for that gene, and the resulting individual will also be homozygous for it.
If the two equivalent genes differ (for example, those determining black coat pigment: the father passes the black gene C, and the mother passes a modified, e.g., Himalayan gene cs), then the resulting individual will be heterozygous for that gene (Ccs). In such a case, the black gene is dominant, and the animal will be phenotypically black but will carry the recessive (hidden) Himalayan gene.
If such a heterozygous animal—a black male carrying the Himalayan gene—is mated with his own sister, also black and also carrying the hidden Himalayan gene, their litter will show segregation (see Fig. 1):
- part of the offspring will be black (CC – homozygotes, and Ccs – heterozygotes),
- and another part will be Himalayan (cscs – homozygotes).
This occurs because when this black male is mated with his black sister, their sex cells—the gametes (sperm and egg), each carrying a single set of genes—will meet in all four possible combinations: C from the male and C from the female; C from the male and cs from the female; cs from the male and C from the female; cs from the male and cs from the female. Consequently, with a probability of 1/4, two Himalayan genes (one from the father and one from the mother) will meet in a single zygote and produce a homozygous Himalayan genotype—and therefore a Himalayan phenotype in part of the kittens.
Another part of the litter, also with a probability of 1/4, will be homozygous for the solid black gene (CC). Thus, the genetic essence of inbreeding comes down to the process of dividing a population into lines with different homozygous genotypes. Since under inbreeding the genes that were previously in a heterozygous state become homozygous, in the next generation—when homozygous animals of the same color are mated—no segregation will occur. In this way, inbreeding reveals hidden traits, fixes desirable traits across generations, and creates stable genetic lines.
The fastest practical way to increase homozygosity in higher animals is mating full siblings (sibs) who share the same father and mother (and it makes no difference whether these sibs are from the same litter or from different litters), as well as mating father–daughter or mother–son. The opinion sometimes expressed among non‑professionals that “a father can be mated to a daughter, but a mother cannot be mated to a son” is a myth with no basis; the effect of these two types of inbreeding is absolutely identical.
If such close inbreeding (incest breeding) is carried out for 16 generations in a row, a 98% homozygosity across all genes is achieved, and therefore, due to the absence of segregation, all individuals in these litters become almost identical in both genotype and phenotype—all offspring look like twins. Of course, in practice, much milder levels of inbreeding are usually used, and accordingly, the degree of homozygosity achieved is significantly lower.
It should be noted that mating third cousins (a shared great‑grandfather) leads to an increase in homozygosity of only 2% even after an infinite number of generations. Therefore, inbreeding of this type is fundamentally different from closer degrees of inbreeding and essentially does not achieve the goals set for a system of related matings. Breeders who, for one reason or another, avoid inbreeding need not fear mating third‑degree relatives.
Determining the Degree of Inbreeding and the Inbreeding Coefficient
Based on an analysis of an animal’s pedigree, one can qualitatively assess the degree of inbreeding, which is expressed by how heavily the pedigree is saturated with common ancestors (names) on the maternal and paternal sides. According to Shaporuzh Roman numerals denote generations (lines of ancestors), counting the parents as the first generation, grandparents as the second, and so on.
If a common ancestor appears in both the maternal and paternal parts of the pedigree, then the number of times it appears in the maternal line is written first, followed—after a slash—by its occurrence in the paternal line.
Thus:
- son × mother is designated as I–II (Fig. 2a),
- father × daughter as II–I (Fig. 2b),
- brother × sister as II–II (Fig. 2c, d),
- grandfather × granddaughter as III–I (Fig. 2e),
- first cousins × first cousins as III–III (Fig. 2f), and so on.
Inbreeding types such as father × daughter (II–I), son × mother (I–II), and brother × sister (II–II) are considered very close inbreedingor incest breeding.
Close-relative matings such as II–III or III–II are classified as close inbreeding.
Matings like III–IV, IV–III, IV–IV, II–V, etc., are considered moderate inbreeding.
Due to their negligible genetic effect, more distant inbreeding combinations can, in practice, be disregarded as true inbreeding.
Owners of high‑quality purebred cats, upon examining a pedigree, can therefore independently assess how frequently and to what extent inbreeding was used by the breeders involved in producing that animal.
The Inbreeding Coefficient
To quantify the degree of inbreeding in an animal’s pedigree—and thus determine the extent of its homozygosity—the English geneticist Sewall Wright introduced the concept of the inbreeding coefficient (F).
It is calculated as follows:
- Count the number of generations (arrows, as shown in Fig. 2) from the common ancestor A to the father of the animal (p).
- Count the number of generations from the same ancestor A to the mother (m).
- Add these numbers together and then add one:
n = p + m + 1 - The inbreeding coefficient of the animal (proband X) based on this ancestor is:
Fₐ = (1/2)ⁿ
If the parents have multiple common ancestors, the individual coefficients are summed:
FX=∑Fi=∑(1/2)n
where i represents the shared ancestors (A, B, C, etc.).
Examples
1. Mating full siblings (II–II, Fig. 2c)
In the pedigree, the distance (path) from the grandfather to the father is 1 generation, and from the grandfather to the mother is 1 generation.
Thus:
n=1+1+1=3
So:
Fb=(1/2)3=1/8
But they also share another ancestor—the grandmother.
The path from her to the father is also 1 generation, and to the mother also 1 generation, so:
Fa=1/8
Total inbreeding coefficient:
FX=Fa+Fb=1/8+1/8=1/4=0.25
2. Mating grandfather × granddaughter (III–I, Fig. 2e)
On the paternal side there are 0 generations, on the maternal side 2 generations:
n=0+2+1=3
Thus:
FX=(1/2)3=1/8=0.125
3. Mating first cousins (III–III, Fig. 2f)
The common ancestors of their offspring are the great‑grandfather and great‑grandmother.
From the great‑grandfather to the father: 2 generations
From the great‑grandfather to the mother: 2 generations
n=2+2+1=5
Thus:
Fb=(1/2)5=1/32
The same applies to the great‑grandmother:
Fa=1/32
Total inbreeding coefficient:
FX=Fa+Fb=1/32+1/32=1/16=0.0625
From existing pedigrees, every breeder or owner of a purebred animal can calculate the inbreeding coefficient of their cats, or of their parents and ancestors—if inbreeding was used in their breeding history.
As already mentioned, this coefficient is an indicator of the degree of purity (homozygosity) of the lines represented in the pedigree, as well as an indication of the possible prepotency of the corresponding sires and/or dams (see below). The higher an animal’s inbreeding coefficient, the more of its genes are in a homozygous state. Naturally, as follows from the formula, the inbreeding coefficient cannot exceed one.
Inbreeding Depression and Heterosis
Why then, in human societies across most civilizations, is incest either strictly forbidden or strongly condemned? Moreover, why in the wild is everything organized in such a way that animals tend to avoid inbreeding and excessive homozygosity?
The main reason is that for successful survival and adaptability to constantly changing environmental conditions (including social and economic conditions in the case of human society), a population requires genetic diversity. In a diverse population, there will always be individuals best suited to a particular situation; however, situations change, and different individuals—those with different traits—rise to prominence.
A uniform population, even if well adapted to its current conditions, is ultimately doomed to extinction if the environment changes. Thus, the drive to maintain diversity is necessary not for individual animals, but for the species as a whole, enabling it to meet the varied and shifting demands of the environment and historical processes.
But there is also a direct danger of inbreeding for individual organisms. Throughout the life of an animal (including humans), as well as across generations, mutations—changes in genes—occur from time to time. Most mutations are harmful, and if they manifested immediately, they would cause death (lethal mutations), deformities, behavioral abnormalities, inability to adapt, etc.
However, most mutations are also recessive—meaning that when paired with a normal gene, thanks to the diploid (double) gene set, they do not manifest in the heterozygous state.
Every heterozygous individual carries a “load” of such harmful but hidden mutations—including every human being.
Inbreeding creates homozygosity, including homozygosity for harmful genes, and therefore leads to their phenotypic expression in the offspring: prenatal or early death, deformities, reduced vitality, and so on. This phenomenon is known as inbreeding depression.
Why, then, do breeders still use inbreeding so often?
Given the risks of inbreeding depression, why do breeders who develop and refine domestic animal breeds so frequently use inbreeding and are not afraid of it?
The answer is simple:
Breeders do not aim to maximize survival, robustness, and high quality in all offspring. Instead, they select the best and remove (for example, by neutering) those that are defective, weak, or simply possess undesirable traits.
Thus, during inbreeding—when the genotype is broken down into homozygous lines—harmful recessive genes reveal themselves and are eliminated from further breeding, while all “good” genes and valuable gene combinations become fixed in a homozygous state and preserved for future generations.
Therefore, inbreeding in the hands of a breeder is a method to:
- remove the load of harmful hidden mutations from the genotype
- create stable combinations of valuable genes and traits
Additionally, homozygosity creates the needed uniformity required for establishing any new breed, ensuring that the desired traits are consistently expressed in the offspring.
It should be noted that as inbreeding and selection of the best individuals progress—and the load of harmful recessive mutations is reduced—the danger of inbreeding depression in subsequent close matings of selected inbred animals diminishes.
Heterosis (Hybrid Vigor)
When representatives of two unrelated inbred lines are crossed, a phenomenon opposite to inbreeding depression can appear—heterosis, also known as hybrid vigor.
Such hybrids of two inbred animals typically show:
- increased vitality
- faster growth
- higher fertility, etc.
This occurs because when crossing two inbred lines, which have been selected and are therefore similar in the essential breed traits, homozygosity for these desirable traits is preserved in the offspring, while the remaining harmful mutations that were not eliminated become heterozygous and therefore recessive—meaning they do not manifest phenotypically.
Practical Purposes of Inbreeding
Let us review and summarize the tasks performed and the goals pursued by livestock breeders—including cat breeders—when using inbreeding.
1. Creation of a new breed or a new subgroup within a breed
When developing a new breed or a new line within an existing breed, inbreeding is essential.
To increase genetic diversity and introduce new traits, a breeder initially crosses animals of different breeds. The resulting hybrid offspring are heterozygous and therefore will display segregation in subsequent generations.
To fix desirable combinations of traits, breeders use primarily close inbreeding, such as brother × sister, father × daughter, and mother × son.
As a result, uniform families are created and the traits become consistently expressed.
Among inbred offspring, very strict culling is carried out—up to 80% of animals may be removed due to defects, weakness, or deviation from the intended standard.
Figure 3 in the original article gives an example of very close inbreeding used in producing lilac‑point Himalayan cats in the English cattery Mingchue.
A chocolate‑point queen, Snuff, was produced by mating a seal‑point male Tromo—who carried both the chocolate and blue genes—from the American cattery Briary with a seal‑point Siamese female, Trivie, also carrying the chocolate and blue genes, from Mingchue.
Snuff (chocolate point) was then bred to her father, Tromo.
Their seal‑point son, Choc, was bred back to his mother, Snuff.
From this mating the first lilac‑point female, Sulatri, was born.
It is noteworthy that Sulatri had a very high inbreeding coefficient: 3/8 or 0.375, even higher than that produced by full sibling mating.
2. Test inbreeding for identifying carriers of undesirable genes
Very close inbreeding is also used when it is necessary to identify carriers of lethal, semi‑lethal, or other undesirable genes.
This is called test inbreeding.
Based on the results of test inbreeding, breeders may decide either to:
- remove such animals from breeding, or
- use them to produce homozygous offspring (through inbreeding), with subsequent elimination of the defective kittens and selection of the healthy ones.
This approach allows breeders to purify the genotype of harmful genes.
3. Producing a highly prepotent sire
Another purpose of inbreeding is to develop a highly prepotent sire—one that consistently transmits his traits to his offspring.
It is well known that inbred animals generally exhibit greater individual prepotency compared to animals produced from unrelated matings.
For this reason, a widely accepted practice in animal breeding is to mate an inbred male with non‑inbred females.
This practice is known as top‑crossing.
A good inbred sire, while transmitting his selected qualities to his offspring, can significantly improve the entire breeding stock and give the cattery its distinctive look and type.
4. Development of inbred lines to produce heterosis
Sometimes breeders intentionally create multiple inbred lines in their catteries so that, later, by crossing representatives of different lines (often in cooperation with other catteries), they obtain the heterosis effect—vigorous, large, hardy, and fertile offspring.
Conclusion!
To conduct effective breeding work—including with cats—it is essential to apply competent systems of selection and mating.
Selection is divided into:
- phenotype selection
- genotype selection
Cat lovers are familiar with various levels of cat shows, where expert evaluation (bonitization) of cats is performed. This is phenotypic evaluation.
Genotypic evaluation, however, must be based on the animals’ pedigrees and on the quality of their offspring.
In general, the true breeding value of an animal can be determined only by evaluating a sire or dam through their progeny.
An animal with an excellent phenotype is not always a good producer.
There is a saying among livestock breeders:
“A good bull is worth half the herd.”
The same applies to stud cats: the quality and “face” of a cattery is determined largely by its stud male (or males, if there is more than one).
Such a stud must be highly prepotent, and therefore breeders should pay particular attention to inbred sires.
Moreover, one cannot build a good cattery merely by purchasing expensive champion and grand champion animals abroad.
It is necessary that within the cattery itself—and from its stud cats used by other breeders—animals of international champion and grand champion level are born.
For this, competent breeding work is required—within which inbreeding plays a significant role.
Conclusion from Vlastar Lux Cattery
At Vlastar Lux Cattery, we believe that responsible breeding is impossible without deep respect for genetics, transparency in methods, and a long‑term commitment to the health and stability of each bloodline. The article “Inbreeding — Pro et Contra” reminds us that no breeding technique is inherently “good” or “bad” — its value depends entirely on knowledge, discipline, and ethical application.
Inbreeding, when used thoughtfully and with professional rigor, becomes a powerful tool that allows breeders to:
- strengthen the type and refine breed‑defining traits,
- identify and eliminate harmful recessive genes,
- develop predictable, stable lines, and
- build a foundation for future heterosis and genetic diversity.
However, it also requires responsibility, careful planning, and a clear understanding of risks. At Vlastar Lux, we use inbreeding only when it serves the long-term health and improvement of the breed — never as a shortcut or convenience. Each pairing is evaluated through pedigrees, genetic logic, and the practical performance of previous generations.
Our mission remains unchanged:
to raise British Shorthairs of exceptional health, beauty, and temperament — cats that reflect both the elegance of their breed and the care invested in their lineage.
We hope this article encourages fellow breeders and cat lovers to look at genetics not as a mystery, but as a guiding compass. With knowledge, responsibility, and love for the breed, we can ensure that every generation is stronger than the last.
Vlastar Lux Cattery
Science-based breeding. Responsible choices. Beautiful results.
