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Eye Color
Image courtesy of the National Eye Institute, National Institutes of Health
• 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.
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
From: Giffiths et al Intro to Genetic Analysis
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.
Two Gene Model
Two Gene Model
• 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.
From: Eiburg 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!
What is a SNP?
A Single Nucleotide Polymorphism1. A marker2. 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.
From: Eiburg et al 2008
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 Non-blue.
Lightcycler Output: eye color
Eye color assay: rs12913832 in HERC2 geneBlue(G)
Non-Blue(A)
We can use Real-time PCR to detect the SNP in the HERC2 gene.
