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Is Blue Really Blue? Understanding Eye Color Eye Color • Eye color depends on the amount of pigment (melanin) in the iris. • Blue= very little melanin. • Green and Gray= intermediate amounts of melanin. • Brown= much more melanin • Albino= no melanin. Image courtesy of the National Eye Institute, National Institutes of Health Different Models • Over time, there have been different models proposed to explain inheritance. • Over the next few slides, we will review – One Gene Model – Two Gene Model – Molecular / SNP Model One Gene Model • Blue eye color is controlled by one gene. • It is a recessive, autosomal gene. • Cause of much consternation, because its by no means completely true. From: Giffiths et al Intro to Genetic Analysis Two Gene Model Two Gene Model Two Gene Model Eye color is controlled by A Blue Gene (BEY2) on Chromosome 15. A Green Gene (GEY) on Chromosome 19. At the Blue locus you could be… At the Green locus you could be Blue (b) • • Brown (B) Green (G) Blue (b) There’s a lot this model can’t explain, beginning with gray and hazel eyes. Or how blue-eyed parents can have a brown-eyed child. And they can. Problem: What is blue? From: Eiberg et al 2008 Model at the Molecular Level • BEY2=OCA2. It’s a real gene. Encodes the P protein that is a precursor to melanin. • Several studies found associations between the non-coding regions of OCA2 and blue eye color. • But they weren’t perfect associations. From: Eiburg et al 2008 Model at the Molecular Level • In 2008, people were surprised to find that there was another gene that was PERFECTLY associated with blue eyes. • It is very close (neighboring) OCA2. • It does not encode melanin. • The nearby gene is called HERC2 and it has a SNP that is perfectly associated with blue eyes in the study population. From: Eiburg et al 2008 Model at the Molecular Level • What’s going on? • Do blue-eyed people have functional melanin genes? • How do you get little melanin in the eyes, but plenty elsewhere? • You need to be able to turn the gene off in the eyes. • REGULATORY GENE! From: Eiburg et al 2008 What is a SNP? A Single Nucleotide Polymorphism 1. A marker 2. A mutation. The single base difference can be the cause of a “broken” gene that leads to a disorder. A block of SNPs that are inherited together is called a haplotype. Model at the Molecular Level • HERC2 binds transcription factors, and is thought to be a gene that regulates OCA2. • This makes sense. And there is only one real distinction– Blue and Nonblue. From: Eiburg et al 2008 Lightcycler Output: eye color We can use Real-time PCR to detect the SNP in the HERC2 gene. Eye color assay: rs12913832 in HERC2 gene Blue (G) Non-Blue (A)