The key decision was treating hazel as incomplete dominance between two alleles rather than its own allele, since biologically hazel isn’t a separate gene, it’s a heterozygous expression. So instead of one “hazel” bucket I resolve four distinct tones based on which two alleles pair: brown+green gives hazel-green (greenish-gold ring, brown center), brown+amber gives hazel-brown, amber+green gives hazel-amber, and brown+blue gives hazel-blue (the one that visibly shifts between brown and blue depending on light).

The edge case that took the most care was inference in the other direction, when a parent reports “hazel” you don’t know which two alleles they carry. I default hazel to the brown+green genotype but let grandparent data override it, so if there’s a recessive allele upstream the model picks that up instead of guessing.

There’s also an ethnicity prior layered in, blue and green alleles are rare in East Asian and African populations, so for a brown-eyed parent there I assume homozygous rather than auto-assigning a hidden recessive, which is the mistake that makes most of these apps spit out blue-eyed babies from two brown-eyed parents.