What birth defect is it when cats have two different colored eyes?

Having two different colored eyes in cats is called heterochromia, not a birth defect. It's a genetic trait, often associated with certain breeds like the Siamese, Turkish Angora, and Odd-Eyed White cat.

While the specific genes responsible for heterochromia are not entirely understood, it's thought to be linked to the distribution of melanin, the pigment responsible for eye color, during fetal development.

It's important to note that heterochromia is a normal variation and not a health concern. It doesn't affect the cat's vision or overall health.