Cat Genetics 1.0: The Basics
Cat owners, cat breeders and veterinarians with cat practices should all have a basic understanding of cat genetics. Intuitively it is quite simple, and it’s all about two. Two parents, but you already know that. Two copies of every gene. Two copies of every chromosome. The number to remember is two. Cat genetics doesn’t get any simpler than this, and what is more, it’s the same simplicity at the root of our genetics. Unfortunately, simplicity plus simplicity plus simplicity …… can quickly give us complexity. But we’ll try to keep it simple.
Think of genes as little biological paragraphs in a book. Each paragraph serves a function; it is biological information with something to say. It is a set of biological instructions. Now think of chromosomes as groups of paragraphs that make up the chapters of the book. Think of the book as all the paragraphs (genes) and all the chapters (chromosomes) put together. Let’s call the book the genome. The genome is a very special book, a recipe book filled with sets of instructions that collectively tell a story. Think of the genome as a recipe book, something like a fusion between ‘The Joy of Cooking’ and ‘The Joy of Sex’, but much more clever. Now think of the genome as a double set of instructions since in genetics, everything (parents, genes, chromosomes) comes in twos.
The cat’s genome tells a very good story. First of all it tells how to make a cat. Then it tells how to be a cat.
Genes are written in a special text called ‘bases’. Bases are the biochemical alphabet of life, and there aren’t very many of them, only four: A, C, G, and T. Simple, right? But remember, simplicity plus simplicity plus simplicity …… eventually gives us complexity. It’s like electricity, where the two conditions of current ‘on’ and current ‘off’ can eventually give us the internet. Or like the 26 letters of our English alphabet that can eventually give us ‘Hamlet’ or ‘Dubliners’.
Back to the bases A, C, G and T. The bases are lined up one after the other, hundreds, thousands, millions of them, like beads on a string and making a special necklace. The necklace is our DNA.
In the laboratory we have learned how to read the bases that make up the genes that make up our genomes. We can read our DNA. We can read our cat’s DNA. We can read the alphabet of life. And even understand a little bit of it’s logic.
Oh what clever apes are we
To know the alphabet of life
To read our genes: A, C, G, T
What power. What humility.
Here is a sequencing profile at the site of a mutation that causes a retinal degeneration disease in cats (PRA-rDAc). The sequencing profile actually represents both copies of the gene in question, one superimposed upon the other. The shaded base (T) indicates that the cat has two normal copies of the gene in question. If a copy of the gene was mutated it would show up as a G.
Back to the genome. The genome contains biological information. How to transfer this information from one generation to the next? Once again, intuitively we know: this time it’s all about sex. Now think of the genome as a deck of cards. Actually a double deck of cards: everything comes in twos, remember? Let’s say that Dad has a genome consisting of only black cards and that Mom has a genome consisting of only red cards. Each parent now shuffles their cards (their genome), and then cuts their deck in two so that only half of their cards will get passed to the next generation. Sex happens. The resulting offspring (kittens in this example) will now have a new genome consisting of a full (double) deck of cards, half of which are black (from Dad), the other half of which are red (from Mom).
The fact that genes come in pairs (one copy from each parent) is a good thing as it allows biology to experiment with genetics and thus allows animals to evolve and become (just for example) cats. But it can also be not so good in that bad copies of genes can stay hidden. A healthy cat can be a carrier for disease genes. If two healthy (carrier) cats are mated they can have kittens that are diseased, and other kittens that are healthy. And that’s at the heart of heredity.
Let’s look at an example of heredity. Consider a black cat (presence of pigment) and a white cat (absence of pigment). Let’s say that white is the recessive trait, i.e. can be hidden, and that black is the dominant trait, i.e. always reveals itself. Let’s call the gene responsible for the presence (or absence) of pigment the gene ‘C’. Furthermore, let’s call the black version of the gene capital ‘C’, and the white version of the gene small ‘c’. Now go back to the idea that you have two parents and that everything in genetics comes in pairs. To have a white cat, we need two white copies of the gene for pigmentation (cc). To have a black cat we only need to have one black copy of the gene for pigmentation, while the other copy can be for black or for white (CC or Cc). CC will give us a black cat that is ‘clear’ (not a carrier) for the white copy of the gene. Cc will give us a black cat that is a carrier for the white copy of the gene.
We now look at three possibilities for mating our black and white cats:
Black X White (‘Clear’ X Affected)
This will give all black kittens. All the kittens will be carriers for white.
Black X White (Carrier X Affected)
This will give black kittens and white kittens, in about equal numbers. The black kittens will be carriers for white.
Black X Black (Carrier X Carrier)
This will give black kittens and white kittens, but more black than white. Some of the black kittens will be ‘clear’, while some of them will be carriers for white.
Once again, genetics doesn’t get any simpler than this. Once again, it can get more complicated. For more comprehensive explanations of basic genetics and the genetics for cats there is a lot of information available only a few taps away on the internet: