Cat Genetics Part Two - Inheritance
This is part of an ongoing series on cat genetics. If you have not yet read part one, it is strongly recommended that you do so before progressing. It is important that you understand how DNA and genes work before continuing.
So, how does inheritance work?
As previously discussed, when a baby is made, it will get half of its genetic make-up from the mother and half from the father. However, as chromosomes swap and change portions of themselves, the genetic material is altered so that each individual has their own unique genetic profile. Even though they are made up exclusively of parental DNA, the whole is more than the sum of the parts.
Gene pools and in-breeding
Because parents pass their genes on to offspring, it is important to have as large a gene pool as possible. It is also important to distinguish gene pools from breed numbers. I have heard it said on many occasions that a breed is healthy because it has a large number of individuals to breed from. However, it is very rare that reference to the available amount of genetic material is made.
Many breeds arose from accidental gene mutations which presented a desirable characteristic in one cat or a related litter. Breeders then work with this mutation to increase the number of individuals in the breed. In the early stages of a breed, they are also widening the gene pool as they bring in cats of domestic ancestry or related and similar-looking breeds. The American Curl began with a single domestic moggy with strangely curled ears, but breeders outcrossed this female, and subsequent generations, to other domestic cats and pedigree breeds, selecting for curled ears in the offspring. In doing so, the American Curl now has a very wide gene pool and relatively few health concerns.
The problem of in-breeding appears to arise, in very generalised terms, from more established breeds. Some years ago, the Ragdoll was imported into the UK. Eight individuals came from America to act as the foundation for UK lines. It is not uncommon today for breeders to claim that they can trace the pedigree of their cats back to the eight individuals, and that only their offspring has been used in subsequent generational breeding. Although the number of individuals has grown extensively, the genetic material available to such cats is still only consisting of 8 individual DNA profiles. Therefore, in-breeding is occurring.
It has been identified that the Burmese cat has one of the most restricted gene pools of all pedigree breeds, and yet, there are many many individuals in the breed pool. This is why it is crucial for breeders and the discerning pet buyer to thoroughly research pedigrees and lines prior to mating or buying a cat.
This clearly means that pedigree cats are bad
Actually, it doesn’t. In-breeding is not confined to deliberate matings organised by breeders. The vast majority of domestic/moggy cats are in-bred to a great extent. Study any local cat population, and it is clear that there are only one or two dominant males. These males will claim mating rights to any and all females who are cycling and ready for mating. Although it is clear that less dominant toms do mate the females, this is mostly observed to be at the beginning of the female’s oestrus. Cats are induced ovulators, which means that hormones must be at a certain level before eggs are released. These hormones increase with mating. So it is unlikely that a female will become pregnant on first mating if left to her own devices in an uncontrolled situation, as males will mate as soon as the cycle begins. In more controlled environments, one mating is sometimes all that is needed if the male is able to mate the female at the right point in her cycle.
Even though less dominant males will mate the females initially, the dominant male will normally complete the mating when the female is ready to release her eggs, meaning that the probability of his paternity is vastly increased.
Cats by nature are incestual, in that they will happily breed with related family members if they are receptive. When the kittens of previous matings mature, it is not uncommon for father or grandfather to mate daughter, son to mate mother or brother to mate sister. This happens to a large extent in uncontrolled local populations, until most un-neutered cats will be related in some fashion.
Inheritance and genetics
Different characteristics can be inherited in different ways. Sometimes, a single gene codes for a single characteristic, but rarely is nature so simple. Normally, characteristics or conditions are coded for by polygenes, a term which simply means that more than one gene is responsible for the presence of a particular feature. Therefore, even when a gene is identified as part of the cause of a problem, it doesn’t always preclude the cat from suffering the problem with a different cause. Breed-specific genes have been identified for hypertrophic cardiomyopathy in cats. However, even if a cat has tested DNA negative for the identified gene, it does not mean that they are immune to HCM. It simply means that the identified gene is not the initiating factor.
Alongside this runs the idea that many different genes can have an impact to a greater or lesser degree. For example, the genes for cat colour are, for the most part, clearly identified, but we do not yet know why some red cats are very rich in colour and why some do not appear so vivid. We do not know why some cats have very blue eyes and why some appear greeny-blue and washed out.
Dominant and recessive
As well as the complexity of mono vs. polygenes, the issue of dominance must also be considered when exploring inheritance possibilities.
As previously mentioned, each chromosome has a corresponding paired chromosome in each cell. This means that you effectively have two copies of each gene. But the copies can vary.
Some conditions require one copy of a gene to be present to express, and others require two copies. Some genes are also dominant over others, in that, if even one of them is present, it will express its characteristic regardless of what exists on the other chromosome. This type of gene is called a dominant. Polycystic Kidney Disease in Persians is a dominant condition, meaning that, if a cat inherits the PKD gene from just one of its parents, it will have the condition.
Contrastingly, the colour cream is recessive. This means that two copies of the gene must be present for the characteristic to express. They must inherit one from each parent in order to appear cream.
When breeding, it is important to fully understand the conditions and characteristics you are working with. It can be argued that sometimes, in-breeding is necessary to either strengthen one characteristic or minimise another, and while it can be successful, you must understand completely whether the risk of increasing negative characteristics outweighs the potential gain. It is important to note whether cat-specific problems are dominant or recessive. If dominant, they are easier to selectively breed against, but the discomfort of any cats who have the undesirable trait must balance this. For recessive conditions, there is less of a likelihood that they will manifest, but they cannot as easily be bred out as they can lie hidden and dormant for many generations before resurfacing.
Breeding cats should not be undertaken lightly.
Please note: In-breeding practices and health conditions are not limited to only the specific breeds mentioned. Pets4Homes in no way suggests that any mentioned breeds are healthier or unhealthier than those not mentioned, and do not suggest that in-breeding is either positive or negative. The breed examples used are based upon the personal knowledge of the author and are in no way indicative of the health or ethics of cats and breeders respectively.
Cat Genetics Series
1)Cat Genetics Part One -What is a Gene?
2)Cat Genetics Part Two -Inheritance
3)Cat Genetics Part Three -Coat Colour
