The Fabric of Our Love

In the last 150 years, the movement of animal fancy has prompted an explosion of new breeds of our much-loved pets. Whilst mostly escaping from breeding for over-exaggeration of certain traits such as those seen in pedigree dogs, cat-fancy has still been somewhat prevalent. Unlike many dogs –take the difference between the great Dane and chihuahua and compare both to the wolf for example –cats have retained their looks and behaviour from their wild ancestors, the African wildcat. Indeed, there has been speculation that the domestication process has never truly occurred in the cat, or that it has only occurred since cat fancy became popular. Nevertheless, breeders have been able to create a huge range of patterns, colours and pedigree breeds whilst maintaining a wild cat in a (sometimes) more tame form.

The dawn of our knowledge of genetics and inheritance began in the 1850s, with the pea plant breeding experiments run by the Austrian monk Gregor Mendel. Although Mendel recognised that traits were being passed down, he was unsure what controlled them. We now call these genes. A gene is the basic unit of heredity and alleles are different variants of a gene. Over the course of his experiments, Mendel crossed thousands of pea plants together and recorded the outcomes of the progeny each time. Analysing different traits and breeding differing plants together yielded interesting patterns –where there was not a ‘blending’ effect of traits as was believed at the time, but all first-generation (F1) offspring would show one of the traits and not the other. An example is the shape of the seed, breeding a rounded seed pea plant and a wrinkled seed pea plant would yield offspring that all had round seeds. Thus, he called these alleles ‘dominant’ and those that were not displayed ‘recessive’. It was only upon the breeding of two of the F1 together that Mendel was able to get the recessive trait to reappear in the next generation. He noticed that generally, the offspring from these crosses would show round or wrinkled seeds in a 3:1 ratio.

However, there was much more going on than meets the eye. Each seed had its own specific genotype for the round/wrinkled trait consisting of two alleles. Where two alleles are the same, these individuals are called homozygotes. Where the two alleles differ, these are known as heterozygotes. As such, the actual genotype ratio differed from that of the phenotype. The grandparents of this generation were homozygous for both the dominant (which we will call R) and recessive (which we will call r) alleles. A cross between the rounded parent (RR) and the wrinkled parent (rr) would then yield only one genotype for that trait (Rr –a heterozygote). Upon crossing two of these heterozygote offspring together, you can see that it immediately becomes a bit more exciting. There then arises the potential for three different genotypes tooccur (RR, Rr and rr) because each offspring will obtain one allele from the ‘mother’ plant and one from the ‘father’ plant. Breeding these offspring together will yield a number of different outcomes depending on the genotypes combined. It is these experiments that have formed the basis of our modern understanding of inheritance. Many of the inheritance rules establishedby Mendel have been essential in creating many of the cat breeds and coat patterns we know and love today.

Spots, rosettes, colourpoints, white markings, shading, smoke and stripes are just some of the coat colours and patterns thatwe can see in our cats, but they all have a genetic basis and some general rules. The genes for coat colour, i.e. black, white, red, or blue are only found on the X chromosome. Male kittens (XY) only obtain their colour genes from their mother as they only have one X chromosome, whilst female kittens (XX) will take one variant from the father, and one from the mother. This is known as ‘sex-linked inheritance’. With some colours, such as lilac and chocolate, the colour genes must be present on both sides of the kitten’s parentage for it to be either of these colours. However, patterns can be inherited from either mother or father. Most are unable to skip generations, with the exception of colourpoint, where the face, ears, legs and tail are often a colour which has a stark contrast to the body colour. This pattern is seen in breeds such as the Siamese, Oriental and Ragdoll.

Tortoiseshell (otherwise known as calico) cats are perhaps the most interesting from a genetic perspective. Where a female cat inherits an allele for ginger fur and another for black fur, instead of one colour being dominant over the other, both will appear together. This is why the vast majority of tortoiseshell cats are female. Although tortoiseshell males are possible, this is usually due to the effects of having an extra sex chromosome (i.e. being XXY), which is essentially the same as Klinefelter Syndrome in humans.

Although we have established that cats are not as diverse as dogs in terms of their looks, there have been some interesting phenotypic departures from the wildcat. One such difference is seen in the tail, some varieties have been bred to have a residual stump of a tail, often known as a ‘bobtail’. These breeds, such as the Japanese and American bobtails are known for their affectionate and laid-back natures, the stumpy tail does not seem to affect their fun and games. In these cases, the bobtail has occurred due to a natural mutation and as these breeds have been derived from feral stock, they have excellent genetic health. The Manx cat is an ancient breed native to the Isle of Man and a dominant mutant allele within the isolated population has led tomany of this breed being completely tailless, although some have stumps like the other bobtail breeds. However, the gene for a complete tail is retained within the population as being homozygous for the tailless gene tends to be lethal, so it is not uncommon for tailed cats to be born. Unfortunately, in entirely tailless Manx, a condition called ‘Manx Syndrome’, a form of spinal bifidathat often results in dramatically shortened lifespan as well as digestive and bladder problems can occur. Manx breeders are often very careful in which cats they breed from and keep a close eye on tailless kittens (affectionally known as rumpies) before they go to new homes due to this condition.

Some breeds also have variations with their facial features, one example beingthe ears. The American curl has a quirky look, with tufty ears that do indeed curl backwards. These cats have a fantastic, bubbly personality and are often described as more dog-like in their devotion to their owners. This dominant mutation originated in just one moggy, which all curls can trace their lineage back to. Despite this, the dominance of the trait has allowed a large genetic pool to be established within the breed, meaning it has both a wonderful personality and fantastic genetic health. Where the curl’s ears are folded back, the Scottish fold’s are flattened to its head, making it look like a cuddly toy. The allelesresponsible for the fold are dominant to those which give normal ears, but the extent to which they are is debated. It is thought that the level of gene expression will dictate whether ears are folded or not. As such, it is suspected that it is possible to have a normal-eared cat that carries the fold variant of the gene. The fold gene is responsible for a condition called osteochondrodysplasia which causes joint degeneration and bone and cartilage malformation. Where

Another controversial breed is the munchkin, which, due to a dominant mutation that causes a condition similar to achondroplasia (dwarfism in humans),has considerably shorter legs than other cats, essentially making it look like a feline basset hound. It is a relatively new breed, developed in the 1980s and is only accepted by two cat showing organisations worldwide because of the view that the breed is unethical and riddled with health issues, especially with their joints. As homozygotes for the mutation are completely inviable, domestic cats are encouraged to be used as outcrosses, so that offspring will have at maximum one munchkin allele. Cats whichare homozygote recessives will have legs of a normal length.

Perhaps the most famous example of the facial difference is in the Persian and its shorthaired variety the exotic. The instantly recognisable flattened snout coupled with large expressive eyesis reminiscent of dog breeds such as the pug and bulldog, giving these cats a clownish look. These flatfaced or brachycephalic breeds have arisen due to a mutation which affects the conformation of the skull. Over the last few decades, Persians have beenselectively bred to give them a flatter face, which are prized in the show ring but, as with pugs and bulldogs, severely obstructs breathing and the ability to lead a normal life. After the fallout of the programme ‘Pedigree Dogs Exposed’ which caused questions to be raised about the similar issues the modern Persian faces, there is now a push for Persians to have more moderate muzzle lengths, similar to the original breed.

Not all breeds have been developed based on skeletal differences, however. A mutation in the gene which causes the Devon rex to have a short, wavy coat is also responsible for creating completely hairless cats. This mutation has been fixed to create hairless breeds such as the Sphynx. The hairless allele is known to be incompletely dominant over the Devon allele –this means that the hairless gene is not expressed as strongly as it would be if it had complete dominance. As such, some Sphynx cats may have some downy hairs. In addition to this, the allele which provides the Selkirk rex with its long, luscious and curled coat also has complete dominance over both of these variants as well as that for a normal coat.

There is also the Cornish rex, which has a similar waved coat to the Devon rex, but this is a result of a different mutation,a deletion (where some of the DNA is accidentally removed) on one of its chromosomes. Cornish rexes only have one layer to their coat, whereas most breeds will have three. Due to this, they are extremely soft but also tend to lose hair. In contrast is the impressive coat of the LaPerm, a breed which originated from a natural mutation in pest control cats in the 1980s. This breed was allowed to naturally propagate for ten years until a proper breeding programme was established. LaPerms exhibit a wonderful long coat of a mixture of waves and loose and tight curls. Wirehaired cats have also been produced the American wirehair arose from a mutation in a litter produced by two American shorthair parents. However, it is thought to be exceptionally unique as there are no reports of wirehair cats anywhere else in the world. Development of the breed produced fur has a texture similar to that of wirehaired terriers. It also has charming crinkled whiskers.

A mixture of mother nature and efforts by breeders have provided the diversity in colour, pattern and breed that we see in cats today. In terms of genetics, the cat world provides a number of interesting examples and here we have only just scratched the surface. Despite the efforts of humans to tame them, cats still resemble their ancestors fairly closely, retaining hunting instincts through play and catching unfortunate birds and rodents that dare cross their paths. Regardless of breeding, you can take the cat out of the wild, but you cannot take the wild out of the cat.

Composed by,

Thea Mainprize, Undergraduate of Zoology