Not two irises are alike, including the irises of identical twins and even the two irises of the same individual. This uniqueness of the human iris is used in iris scans, similar to fingerprints. The variation of iris pigmentation is caused by its complex structure and coloring pattern. A striking example of the dissimilarity of the iris in one person are people with heterochromia.
https://en.wikipedia.org/wiki/Heterochromia_iridum
What Is Heterochromia?
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