Photo CC-BY Jon Chapman

Dear Most Esteemed and Knowledgeable Kitties:

My cat just had kittens… one black and white, one black, and one calico. Both parents are black and white. Can I still assume that my black and white male is the father?

~ Dawn

Thomas: Oooh, Mama’s even more excited than we are to answer this question. Why? It’s all about genetics, and Mama’s a huge science nerd!

Bella: Can I answer first? Can I please? Pleeeease?

Tara: Fine with me.

Thomas: Okay, Bella, go ahead.

Bella: Well, Dawn, you don’t say what color your mama cat is, but if mama cat isn’t calico, tortoiseshell or orange herself, then the father would have to be an orange, orange and white, or cream-colored cat.

Tara: Why is that? It’s all about genetics. You see, the genes for fur color are carried on the X chromosome, and …

Thomas: Wait, hold on a minute. For those of you who don’t remember high school biology, there are two chromosomes that determine a mammal’s sex. The X chromosome says “female” and the Y chromosome says “male.” If a cat inherits and X and a Y chromosome, he’ll be male, like me.

Bella: And if a cat inherits two X chromosomes, she’ll be female, like me.

Tara: That’s the basics. Now, on to fur color. Like I said earlier, the genes for fur color are carried on the X, or female, chromosome.

Thomas: And that means female cats carry two genes that determine fur color, while male cats only carry one.

Bella: Now, on to gene properties. Some genes are dominant, which means they express themselves when only one copy of the gene is present; while some genes are recessive, which means a cat has to inherit two copies of the gene — one from each parent — for that gene to be expressed.

Tara: It just so happens that the orange gene is dominant, which means as long as a cat inherited even one copy of that gene, it will be expressed in the fur color.

Thomas: But interestingly enough, the black color gene is also dominant, which means that a cat needs to inherit only one copy of the gene for that color to be expressed.

Bella: What does that mean about inheritance? Well, let us show you through this thing called a Punnett Square. Again, if you remember high school biology, you may have seen these before.

This Punnett Square demonstrates at a very basic level how calico colors are inherited.

Tara: So, what you see here is that the mother cat has two dominant black genes on her X chromosomes, whereas the father cat has only one color gene because he only has one X chromosome. The Y chromosome is white to indicate there’s no fur color inheritance from that chromosome.

Thomas: So, when you look at this chart, you would see that an orange cat and a black cat mating would produce calico female kittens and black male kittens. (The inheritance of white fur color happens with a different gene. We’re not going to discuss that because things will get really complicated then!)

Bella: On the other hand, if two black or black and white cats mated, you’d get this Punnett Square.

This Punnett Square demonstrates at a basic level the result of two black or black and white cats mating.

Tara: So, as you can see, it would be impossible for two cats without the orange gene to produce any orange or calico kittens.

Thomas: The even more interesting thing here is that litters of kittens can have multiple fathers, so it’s possible that your black and black-and-white kittens were fathered by the black and white father, while the little calico had a different father. That father would have to be orange, orange and white, or dilute orange (cream colored).

Bella: There’s a whole lot more complicated genetic stuff that explains why calico cats have black and orange spots when both genes are dominant, and we’re not going to go into the full depths of it in this post.  This phenomenon is called “X inactivation,” which means that in some cells the black color gene is turned off and in some cells the orange color gene is turned off. That produces patches of orange fur and patches of black fur.

Tara: This is a really super-simple explanation of the genetics of fur color. The full depth of it is super-complicated and super-fun, and if you’ve enjoyed this taste of fur color genetics, you might enjoy checking out the following links:

• Basic Genetics as Revealed by Cats (Berkeley College course materials)
• The Genetics of Calico Cats (University of Miami) — this goes into more depth about X inactivation and gives a higher-level discussion of how calico color is inherited.
• Are Orange Cats Really That Rare? (a Paws and Effect interview with a college professor who teaches genetics)

Thomas: Do you other readers have any genetics questions we can help you understand? Whether it’s about fur color, diseases, or anything else, just ask. We love to answer science questions because they make Mama super-happy! Also, if you have any comments, please feel free to share them here.