Basic Feline Genetics
Every Breeder and Serious Exhibitor Should Have a Basic Understanding
Understanding feline genetics is critical to breeding and selecting healthy kittens and cats. While the science behind feline genetics is quite complex and new discoveries emerge every day, one doesn’t need to be a cell biologist to understand the basic concepts needed to make good decisions about your next exhibit or breeding pair – understanding just a few terms and concepts is all most people need to know. We will cover on this page basic terminology and concepts, feline health genetics, feline genetic traits, and genetic diversity. For those who want to dive deeper into one or more of these topics there are links provided for additional information.
1. Basic Terms and Concepts
Next time you crack open an egg, stop and think about how that yellow yolk contains all the needed information to develop from a single cell to an adult chicken, with many developmental steps in between. Inside that yolk, known as a cell’s nucleus, are chromosomes. There are two copies of each chromosome, one copy provided by each parent. Each chromosome consists of building blocks of nucleic acids called base pairs, that are organized into segments called genes. These genes contain the recipes cells need to manufacture proteins which are organized and differentiated into more cells, then tissues, organs, and eventually a fully formed, living organism.
Variations on a Theme
Variations in the base pairs and their sequences on the chromosome, also known as mutations, are what produce genetic diversity. Mutations can be good or bad – they produce the variations in coat color and texture, but they also can produce inherited health issues such as polycystic kidney disease (PKD).
In cats, there are 19 pairs, or 38 chromosomes. 18 pairs are autosomes, and one pair is the sex chromosomes – XX in females, XY in males. On these 3 chromosomes are approximately 20,000 genes containing over 3 million base pairs. Approximately 2% of these base pairs are directly involved in protein synthesis, the other 98% are often referred to as “dark DNA”. Dark DNA has no obvious function, though researchers believe they may play a regulatory role in turning certain genes on and off.
Variations in one or more base pairs for a specific gene are called alleles. Since each parent provides one copy of each allele, a cell can have two alleles that are the same, or they can have different alleles for the gene. When the alleles are identical, this is known as a homozygous allele. When they are different, they are heterozygous.
Two Halves of a Whole
When alleles are heterozygous, it means that the cell has two different sets of instructions on how they should manufacture a particular protein. The concept of dominance determines how the protein is manufactured. If an allele is dominant, it will be expressed over a recessive allele. A good example of this is the dilution color trait, which dilutes black pigment to blue (grey) and red pigment to cream. The full color allele is dominant over dilution allele, so if a cat is heterozygous and has one full color and one dilution allele, it will be full color. A cat must be homozygous for the dilution allele to express the diluted hair color. To further the example, if two heterozygous black cats mate, the offspring will be homozygous black, heterozygous black, or homozygous blue.
The only way to determine if a cat is homozygous black or heterozygous black is genetic testing. We already know the blue kittens are homozygous for the dilution allele, because recessive traits must be homozygous to be expressed. Two blue parents will always produce blue kittens, though other alleles such as the dominant white spotting and agouti (tabby) genes and the recessive colorpoint restriction genes will affect how that pigment is distributed in the hair coat. Some traits can be co-dominant, meaning both alleles can be expressed. A familiar example of this is human ABO blood type. If there are two A alleles or one A and one O (absence of either A or B), the blood type will be A. The same holds true for B. However, if the is one A or one B allele, both proteins will be expressed and the blood type will be AB.
It is important to note that this type of inheritance, often referred to as Mendelian inheritance, where a single gene or allele is involved, is not the only type of inheritance. There is also polygenic inheritance, which involves two or more genes. Many diseases are polygenic, as are traits such as size measures or ear and eye shape.
Genetic testing of cats has been available since 1990’s, when research facilities at veterinary schools began offering parentage testing of cats and other animal species. As work on the feline genome progressed and scientists matched specific genes were matched to specific traits and health conditions, breeders began to screen their breeding cats for genetically based health conditions such as PKD and pyruvate kinase deficiency (PK def). For a more in depth look at these and other inherited conditions, please see our Feline Health Genetics page.
WHY EVERY BREEDER SHOULD GENETICALLY SCREEN THEIR BREEDING ANIMALS.
Even if your breeding stock has been healthy for generations, recessive defects can be passed on undetected until two carriers are bred together. The likelihood of two carriers mating increases in populations with low heterozygosity and high Coefficient of Inbreeding (COI). However, genetic screening alone does not guarantee that a kitten will be free of inherited defects. The University of Sydney maintains a database of identified genetic disorders and traits on many species of animals, (including cats, ) and at this writing there were 415 documented inherited defects and traits in cats, with more being identified and added on a regular basis. Commercial genetic screening panels only screen for approximately 40 – 60 inherited disorders. A thorough examination of kittens and breeding cats by a licensed veterinarian is critical in identifying inherited defects and when these defects are being manifested in multiple related animals, an inherited disorder needs to be considered.
Understanding feline genetics can go a long way in predicting what the offspring of two parents might look like, even without genetic testing. For example, two blue point Siamese will only ONLY produce blue point Siamese, because blue, colorpoint, and no tabby striping are recessive genes and require two copies to express. However, a seal (brown) point Siamese and a blue point Siamese could produce all seal point if the seal parent is homozygous, or both seal and blue if the seal parent is heterozygous. If no genetic testing is done, the breeder can only say there might be blue offspring. If genetic testing is done, the breeder can state with confidence that there will be no blue offspring if the seal parent is homozygous.
Coat color is not the only trait that can be screened for. Coat pattern, length, and texture (curly or straight) can be screened for. Non-coat traits such as tail length and polydactyl toes can also be screened for. For an in depth discussion of color inheritance, color descriptions, clues for determining the color of your cat or predicting the color of their offspring without genetic testing, click here.
4. Genetic Diversity
In prior sections, we introduced the terms “heterozygosity” and “coefficient of inbreeding”, both useful measures when determining the genetic diversity of a breeding pair or population. Heterozygosity can be measured as a percentage and percentage of heterozygosity is often part of commercially available genetic screening panels. For example, a genetic screen may return a percentage of heterozygosity of 37%, with a reference value of 35% - 40% for that breed. What this means is that 37% percent of the genes have different alleles from each parent, and 63% of the alleles from each parent are identical. Selecting two parents with higher heterozygosity will decrease the possibility of the parents passing on a genetic defect but will also produce more variance in the appearance (phenotype) between parents and offspring. Lower heterozygosity will produce a more consistent look between parents and offspring but increase the possibility of a genetic defect appearing. T
The coefficient of inbreeding (COI) is a measure of how related a cat’s ancestors are. Because of the way nature shuffles the gene deck when creating an organism’s reproductive cells (gametes), full siblings (both parents are the same) share about 50% of the same DNA. If full siblings mate, that 50% is halved again, producing an offspring with a COI of 25%.
Statistically, what this means is that if a parent has a recessive (one copy) genetic defect, they have a 50% chance of passing it on to each offspring. All animals have recessive defects, so if full siblings are bred, 25% of a parent’s defects will be matched up in their offspring. Recall that there are over 400 identified genetic defects, and only about 15% are screened for. The odds of an identified or unidentified genetic defect paring up in full-siblings is very high. While they may not manifest themselves with easily detectible genetic diseases such as PKD and some forms of HCM, offspring with a high COI tend to have higher infant mortality, shorter life spans, decreased fertility, and smaller size than offspring with low COI.
Breeders need to consider both the percentage of heterozygosity and COI along with the overall health history of the lines under consideration when selecting breeding pairs. While cats with a high COI tend to have a lower percentage of heterozygosity, keep in mind that if a cat with a high COI is bred to a completely unrelated cat, the COI is still zero. If both cats are healthy and have no adverse findings on their genetic screens, they can be bred with the same confidence as two unrelated cats.
While lower heterozygosity would also indicate less genetic diversity, it also statistically reduces the potential number of hidden recessive defects. If two unrelated cats with low heterozygosity with similar phenotypes are bred, there is increased likelihood that type will be set and the offspring more likely to closely resemble their parents.
Keep in mind, though, that just like any statistically based decision, things can randomly go horribly wrong since you are rolling genetic dice, and the result of the roll can never be 100% predicted. However, these measures, along with a thorough understanding of the health of the ancestors and their offspring for as many generations as possible can go a long way in deciding an acceptable level of statistical risk.