I have partial heterochromia iridis, but my identical twin does not. How is that possible?

Nevus... ...Or birthmark - if it happens right over the eye it will involve the iris. A centimeter away, and it's just a birthmark.

Related Questions

I have partial heterochromia iridis, but my identical twin does not. Is this a sign that we are fraternal twins?

Nope! This is a minor somatic difference that does not negate the fact that you two are otherwise identical. You have an expected ~100% identical dna with somatic mutations being the exception that confirms the rule. Read more...