Tuesday, 15 May 2012

Eye Colour in Humans – Not Just a One Gene Affair


Contrary to popular belief eye colour in humans is not just controlled by one gene in our DNA. High school biology teaches us about Gregor Mendel and his theories about inheritance patterns and how they relate to human eye colour. Prior to recent studies conducted into the genetics of eye colour it was thought to be a strictly mendelian trait (White and Rabago-Smith). However it is now known to be the product of multiple genes. This theory is used to explain why eye colour does not comply with Mendelian patterns of inheritance. For example blue-eyed parents are able to have brown-eyed children, which should not be possible in a Mendelian model where brown is dominant over blue (which can only occur with homozygous recessive genes and thus they would not be able to pass on the dominant gene to their offspring).  This model using a single gene is unable to explain the spectrum of eye colour and the fact that eye colour in humans shows both incomplete dominance and epistasis (University of Queensland).  This suggests that there is more than one gene that controls eye colour.

The colour of an individual’s eye is determined by the ratio of two pigments in the iris of their eye. These two pigments are called eumelanin (the yellow pigment) and phenomelanin (the black pigment) (White and Rabago-Smith). These pigments, melanin, and are produced in the melanocytes of your eye (Ibid). Blue eyes arise from low levels of melanin and increasing levels produce the rest of the eye colour spectrum (Stanford University). The amount of melanin in the iris also affects eye colour. The more melanin that is present, the darker the apparent colour of the eye as when light enters the eye it is largely absorbed rather than reflected back as colour (White and Rabago-Smith). So people with lighter eyes have less melanin than people with darker shades. Individuals may have red or violet eyes; this is due to a condition called ocular albinism and is caused by mutations in their gene sequence (Ibid).  

Studies conducted by various institutions including the Institute for Molecular Bioscience at the University of Queensland have shown that there are 16 genes which affect eye colour (Ibid). However most of these genes have only a small effect, the two major genes are HERC2 and OCA2 (Ibid). HERC2 affects the way in which the code of OCA2 is expressed in the DNA sequence because of its position in the DNA (Ibid). Any changes in the sequence of these genes have large impacts on the eye colour of the individual. Changes in the OCA2 gene been shown to account for around 74% of variation in eye colour (Duffy, Montgomery and Chen; White and Rabago-Smith). If both copies of the OCA2 gene are missing it leads to ocular albinism (White and Rabago-Smith). Other genes which effect eye colour include agouti signalling protein, tyrosinase, membrane associated transporter protein, p protein oculocutaneous albinism II and melanocortin 1 receptor (Ibid).

It is clear from the research that has been conducted in this area that the Mendelian model of inheritance is unable to explain the expression of eye colour in humans. There are multiple genes responsible for melanin production in the eye and the main two are HERC2 and OCA2 (Tyler).

Bibliography

Duffy, David L., et al. " A Three–Single-Nucleotide Polymorphism Haplotype in Intron 1 of OCA2 Explains Most Human Eye-Color Variation." The American Journal of Human Genetics (2007): 241-252.
Riedell, Kelly. Genetics, Chapter 11. 20 March 2012 <http://local.brookings.k12.sd.us/biology/Genetics.htm>.
Stanford University. Understanding Genetics: Human Health and the Genome. 2 July 2004. 10 March 2012 <http://www.thetech.org/genetics/ask.php?id=29>.
University of Queensland. No Single Gene for Eye Colour, Researchers Prove. 22 Februaury 2007. 13 March 2012 <http://www.sciencedaily.com/releases/2007/02/070222180729.htm>.
White, Desiree and Montserrat Rabago-Smith. "Genotype–phenotype associations and human eye color." Journal of Human Genetics (2011): 5-7.
Wood, Sarah. Eye colour more complex than we thought. 23 February 2007. 10 March 2012 <http://www.cosmosmagazine.com/node/1062>.

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