Eye Color Inheritance: Why Brown-Eyed Parents Can Have a Blue-Eyed Child

Eye color is a polygenic trait involving major genes like OCA2, HERC2, TYR, TYRP1, SLC24A4, and IRF4. The simplified Mendelian model (brown dominant over blue) approximates population trends but can't predict individual offspring with certainty.

Two-Gene Model (Illustrative)

Imagine two loci with dominant (B) and recessive (b) alleles for brown/blue.

• Both parents are brown-eyed but heterozygous (Bb).
• Punnett square: 25% BB (brown), 50% Bb (brown), 25% bb (blue).

Thus, two brown-eyed parents can have a blue-eyed child if both carry a recessive allele. Real-world genetics involves multiple SNPs with additive effects; polygenic scores explain over 50% of variance.

Beyond Brown and Blue

• Green/hazel: Intermediate melanin plus structural scattering (Rayleigh); GEY locus implicated.
• Albinism: OCA2/TYR mutations → minimal melanin → very light blue or pink irises.

Phenotypic Changes Over Time

Infants often start with blue-gray eyes (low melanin); melanocytes mature over 6–36 months, revealing the final color. Trauma, medications (prostaglandin analogs), and diseases (Horner's, Waardenburg) can alter appearance later.

Genetic testing for eye color is recreational, not diagnostic.