Cat Coats and Genes
Closer to the origin of Tabby cats' stripes
January 19, 2010
Ever wonder where Garfield got his stripes? Maybe not, since he's a cartoon, but he's based on real cats.
Tabby cats sometimes have stripes like Garfield. They may also have other patterns: spotted, ticked, or blotched. All of these patterns are caused by their genes.
Murray is a tabby with
a striped, or mackerel, pattern.
Cat aficionados once thought that all four patterns were controlled by a single gene that comes in four different versions. However, scientists recently found that there are at least three genes that contribute to making a cat striped, blotched, spotted, or ticked. And these three genes all work together to give a final coat pattern.
For example, one gene determines if a cat will be striped or blotched. A second gene can cause spots by turning stripes into spots. So if a cat doesn't have a gene that can lead to stripes, it can't have spots either.
This isn't as confusing as it seems. In fact, it's how a lot of genetics works"Šone gene can affect how another works.
Think about these two genes this way. Imagine that you have a red wall and a wall with red and black stripes. And you have a bucket of red paint.
You can't do much to the red wall. Adding some red to more red gives, well, red. But you could use the red paint to turn the black stripes into spots.
And that is what is happening with these cats. The blotched cat is like the red wall and the spots gene is like the bucket of red paint. Just like adding red paint does nothing to the red wall, adding a spots gene to a blotched cat doesn't do much either. But if you have a striped cat (the red and black striped wall), then you can get spots.
These scientists also started looking for the genes involved in a cat's coloring. They were able to narrow down important regions to three different parts of a cat's DNA. This provides additional strong evidence that more than one gene is involved.
Understanding how a cat's coat is patterned isn't just important to cat breeders. There's a good chance that other animals use similar genes. That means this study may get scientists closer to knowing what causes Spot's spots and Tigger's stripes.
Striped and Blotched: One Gene or Two?
Three genes are involved, but how did scientists come to that conclusion? Basically, geneticists expect certain results when one gene is involved in a trait. When two are involved, they expect different results.
Let's look at the simplest case: one gene with two versions. Remember, genes come in different versions or flavors, just like ice cream comes in chocolate and vanilla. Scientists were studying the gene Tabby and its versions: Striped and Blotched, so we'll use those.
Cats (and people) have two copies of each gene, one from mom and one from dad. That means they can have different combinations of versions. In this case, the following three are possible: Striped/Striped, Blotched/Blotched, and Striped/Blotched.
Another important fact is that some versions are stronger than others. Geneticists call the stronger version "dominant".
In the case of Striped and Blotched, the Striped version is dominant. If a cat has the Striped/Striped combo, it will be Striped. If it has the Blotched/Blotched combo, it will be Blotched. A cat with Striped/Blotched will be Striped because Striped is stronger than, or dominant to, Blotched. I summarize this here:
OK, now we understand four important concepts:
- There are different versions of genes.
- Some versions are dominant to others.
- A cat has two copies of most of its genes.
- A cat gets one copy of a gene from mom and the other copy from dad.
With this information, scientists can begin to predict what would happen when cats with different patterns mate. Let's use the example of a male with a Striped/Blotched combo mating with a female with Striped/Blotched. Remember that Striped is dominant, so both cats have striped coats and maybe look like this:
Each cat will give either Striped or Blotched to their kittens. That means there are four possible genetic combinations a kitten can have. Three combos will give kittens with stripes. The fourth will make a kitten blotched.
Which combo an individual kitten gets is random chance. But the odds are that three out of every four kittens will be Striped. Geneticists use this ratio of 3:1 (3 Striped:1 Blotched) to determine if traits are caused by one gene. If another gene is involved, the ratios change dramatically.
Scientists used these expected ratios to figure out that Striped (or Mackerel) and Blotched are different versions of the same gene, which they call Tabby. So now we know that striped and blotched happen because of one gene"Šbecause scientists got this 3:1 ratio. But what about spots?
There's More to Spots Than Tabby
Scientists also wanted to see if the Spotted pattern was due to the Tabby gene. Like ice cream comes in more than chocolate and vanilla, some genes can come in more than two versions too. So maybe Tabby comes in spotted, striped and blotched versions.
Scientists already knew that the Blotched pattern is caused by the Tabby gene. Additionally, previous studies suggested that Spotted is dominant to Striped and Blotched. So, scientists mated a spotted cat with a blotched cat. Here is what we are working with:
All the kittens had one Blotched version of Tabby from their Blotched parent. And if Spotted was simply another version of the Tabby gene, they all got a Spotted version of Tabby from their Spotted parent.
They all would have had a Blotched/Spotted combo. Because Spotted is dominant, all of the kittens' coats should have been Spotted. In other words, we might have expected this result:
But that's not what they saw. Instead, they got this:
They found that all of the cats had a pattern that was a mix between Spotted and Striped. Striped? But where did stripes come from? We'll get to that in a little bit.
First, the result suggested that scientists were NOT dealing with the simple situation of one gene with two versions where Spotted was dominant. One of two things was going on. Either two genes were contributing to Spotted or a phenomenon called co-dominance was happening.
I told you that one version of a gene is always dominant to the other. While that is often the case, there are exceptions. Co-dominance means that neither version is stronger.
This happens in humans, too. One example is curly versus straight hair. Let's say a person with the Curly version has kids with a person with the Straight version. Their kids will be Curly/Straight and have wavy hair, which is like a combination of both versions.
Similarly, it is possible that the Spotted version and the Blotched version could both contribute to the coat pattern. So, the combo Spotted/Blotched would give the kittens a New pattern.
To test for co-dominance, scientists did another mating. They mated cats that had the New pattern with Blotched cats, which had the combo Blotched/Blotched.
All the kittens would get Blotched from their Blotched parent. The other parent would give Spotted or Blotched. That means half the kittens would be Blotched/Blotched, giving them Blotched coats.
The other half would be Blotched/Spotted, giving them the New pattern. That's what would be expected if the Spotted version were a co-dominant version of the Tabby gene.
Interestingly, they found half the cats were Blotched, but the other half was not simply the New pattern. That argued against co-dominance. Some cats actually had a distinctive Striped pattern.
Back to figuring out where the stripes come from. If Spotted and Blotched were both caused by the same gene, Tabby, where would the stripes come from? The answer is that a second gene must be in play.
Here's what scientists think is going on. The Tabby gene leads to either stripes or blotches. When the Tabby gene is Blotched/Blotched, the cat is simply Blotched. That's what happened in the cats that were Blotched.
However, if the cat is striped, a second gene matters. The second gene modifies the stripes to spots. Spotted cats have at least one copy of the Striped version at the Tabby gene AND a copy of Spotted at the Gene 2 location.
Without the Spotted version of the gene, striped cats remain striped. So, the striped cats in the previous section must have the Not-Spotted version of Gene 2. The chart below shows what gene combinations lead to what coat patterns.
Scientists also wanted to see what causes the Abyssinian, or Ticked, pattern. They mated Ticked cats with Blotched cats and checked their kittens and grandkittens. From the initial experiments, it looked like Ticked could be a version of the Tabby gene.
However, when they took a closer look at the DNA, they found that Ticked is not in the same place as the Tabby gene. It's in a different region of DNA. So, it must be a different gene.
So, what are all of these genes actually doing? Well, I've been using "gene" rather loosely. My use suggests that scientists have found the exact genes that cause different coat patterns. In reality, they've narrowed it down to regions of DNA, and those regions contain many genes.
One of the genes in the Tabby region is important for transporting pigment, which may be important for patterning. They don't speculate on which genes Ticked might actually be, but they do provide a list, which may inform scientists in the future who are trying to learn more. As for Spotted, they didn't figure out a region for it.
While this paper does not solve the mystery of which gene causes Tabby coats, it does get scientists closer to the truth. Now, they know that they are looking for three genes, instead of one. Soon, we'll know exactly how a cat gets its stripes.