Why do some cats have markings underneath their nose (moustache-like)?

[some_neurologist]

I noticed some bicolour cats (mostly black and white) have large splotch under their nose, why is that and what it is called?

 

[di and bob]

We all call it a mustache, it seems! It begins with the 7 genes that determine coat color, only 7! These 7 genes only determine cat coat colors, other genes determine cat characteristics, such as length of fur, body build, etc. In the 'mustache' case, it is the S allele that is responsible for mustaches, mittens, and spots. A dominant S gene creates white spots anywhere on the body that covers the cat's true color, whatever it is. It can even create totally white cats (which will have green or yellow eyes, NOT pale or blue, which is the W gene) One giant spot! A recessive S gene means absolutely NO white fur anywhere.
There you go, now you know the S gene is responsible for those adorable mustaches!