Understanding inbreeding: history, health and FIP

An introduction to inbreeding in pedigree animals and its implications: the positive and the negative.

Inbreeding example

Inbreeding: necessary or avoidable?

Inbreeding is a very emotive topic in all species of animal, but is particularly relevant in the pedigree dog and pedigree cat realms. For the purposes of this article we will focus specifically on pedigree cats and kittens, and primarily the British Shorthair breed.

It is important to keep a balanced view on inbreeding. In the wild, there will always be some degree of inbreeding since cats will naturally breed with their close relatives if they come into contact with each other when the female is in heat. Therefore, some degree of inbreeding is natural.

Inbreeding in pedigree breeds

Inbreeding in the early stages of the establishment of a pedigree breed is essential as it is the only way ‘fix’ certain traits. In the British Shorthair cat, the short, dense coat, the beautiful colours and the large cobby body type are all traits that had to be ‘fixed’ in the early days by inbreeding.

Because of the nature of pedigree animals, all British Shorthairs are likely to be related if you trace back the lineage far enough - since all of them come from the same foundation cats. In the same way, if we traced back our own lineage far enough we would also find that we are related to a vast number of people.

Therefore, inbreeding is, to a certain extent, natural and is a vital tool in developing a breed of animal with a specific ‘look’. In order to maintain and improve the breed we must carefully select which animals to breed together, in a way that enhances their features with each generation wherever possible.

Inbreeding Coefficients 

The Inbreeding coefficient represents the probability that two copies of the same variant at a gene have been inherited from an ancestor common to both sire and dam. Inbreeding coefficients can be calculated using various mathematical approaches. Most breeders use software to automatically calculate the inbreeding coefficient.  The GCCF describes the coefficient levels as follows: 

  • 0-10% = low level inbreeding coefficients, which are 'ideal' 
  • 10-20% = fair level of inbreeding which is 'reasonable and acceptable' 
  • 20-25% = close level of inbreeding, 'close matings approaching the higher end of what breeders should normally consider' 
  • 25-40% = high, 'should only be used by experienced breeders for very specific reasons' 
  • 40%+ = not ideal, 'the welfare and health of cats with such levels of inbreeding is highly likely to be compromised and such matings should not be undertaken'

All of our kittens are in the lowest range. 

Inbreeding and genetic diversity

Inbreeding is the fastest way to develop specific traits in lines of cats, and it remains a useful process for all breeders. Nevertheless, it has some clear and real drawbacks. Inbreeding reduces genetic diversity. When an egg or a sperm is formed, one cell from the mother or father divides into 2 eggs, or 2 sperm. In that cell, the pairs of chromosomes divide into individual chromosomes and each egg or sperm receives one chromosome from each pair. When we inbreed, the number and variety of different chromosomes is reduced. It is dramatically reduced where very close relatives are mated, like full siblings or parents to children. It is less reduced where cousins or great grandparent to great grandchild are mated.

It is this reduction in diversity of genes that leads to the uniform appearance being created. The down side of this process is that, after a certain point which cannot be pre-determined, the reduction in genetic diversity starts to impact upon the health of the animal.

Initially, the kittens and cats will seem low in spirit, slightly depressed, and this is known as inbreeding depression If the line is continued then the cats and kittens produced will go on to develop genetically derived illnesses. This is because if there is any mutation or problem within the genetics of the line, it is multiplied so many times that it can become a major issue.

Inbreeding and health

An additional factor is that cats with high inbreeding coefficients are known to have significantly weaker immune systems. This is thought to be caused by the reduction in genetic diversity, but the link is not yet fully understood at this time.

Inbreeding and Coronavirus and FIP

inbreeding in cats coronavirus and FIP
Feline Coronavirus 

A specific illness known to be affected by high levels of inbreeding is Feline Infectious Peritonitis, also known as FIP. FIP is a lethal illness caused by Feline Coronavirus, or FCoV. The virus is spread via the fecal-oral route between cats, so cats in multi-cat households where they share litter trays are at highest risk. Good litter hygiene in multi cat households is essential. You can read about good litter hygiene here.

All of our cats have tested negative for FCoV. 

The vast majority of cats will become infected with Coronavirus at some point in their life, and around 90-95% of those cats who are infected will recover from it completely, usually in around 9 months. A small number of cats will show no real illness or symptoms but will become persistent shedders. This means that they will shed Corona virus in their feces for the rest of their lives, either constantly or intermittently.

The remaining c.5% of cats will go on to develop FIP, usually within the first year of contracting Corona virus. Whilst much research has been done on FIP, we still do not fully understand the condition. The research so far strongly suggests that:

  • cats who are more in-bred and therefore have lower genetic diversity are thought to be at higher risk of developing FIP
  • litter hygiene is also important as poor litter hygiene promotes the spread of Corona virus, particularly where different cats share litter trays. Our cats never share a litter tray: they all have their own that is not accessible to any other cats
  • overcrowding is thought to play a role as well because it causes stress which weakens the feline immune system. It also contributes to poor litter hygiene, because multiple cats access the same litter tray.  

Read more about Feline Coronavirus and FIP.

Inbreeding and fertility

Inbreeding is also known to affect fertility. Endangered wild cat populations have been studied in some depth and the findings demonstrated that these populations have a high level of inbreeding. This was found to have a two-fold effect on the fertility of the males studied: first, they had greatly reduced libidos, so did not respond to females in the are as often, or at all, and secondly, their sperm were very deformed - most of the sperm were found to have two tails rather than one, with the effect that they could only swim in circles. This meant that the sperm were not capable of reaching the egg and achieving fertilisation. Lower libido and fertility are both signs of what is known as ‘inbreeding depression’.

Our approach

We aim to avoid inbreeding as far as possible. This is for two main reasons:

  1. We aim to produce wonderful kittens that will make fantastic, beautiful family pets. For us, the primary concern is health - a kitten that goes on to develop an illness will bring misery to the family, so to us health is paramount

  2. In addition to this, in the British Shorthair breed there are a very large number of cats. This means that it is not necessary to use inbreeding to ensure the survival of the species

That said, it is almost impossible to have an inbreeding coefficient of 0%. Most of our kittens are in the region of 1-2%, which is a relatively tiny percentage. Some of our kittens in the rarer colours might have higher inbreeding percentages, but all are in the lowest, 'ideal' category of 0-10%.

If you have any questions about any of our cats, or would like to provide a home for one of our kittens, please contact us for the latest availability.


- Extremely flat faced cats and their health problems 

- Swimmer Syndrome and how to fix it

Fading Kitten Syndrome and blood group compatibility

- Litter tray hygiene and cat health

- Why all of our kittens are officially registered