Perceived eye, or iris colour varies as a continuous from blue to brown/black and is determined by the melanin pigment, quantity, packaging and type produced in the melanocytes of the anterior border layer and stroma of the iris. Brown eye colour is the result of abundant melanin pigment content and a higher ratio of pheomelanin to eumelanin. The number of melanocytes is not a determining factor. Blue eye colour results from the scattering of most blue wavelengths to the surface by collagen fibrils, in the presence of minimal melanin pigment. (1)

Once thought to be an example of simple Mendelian inheritance where brown colour was dominant and blue colour recessive, iris colour is now known to be a polygenic phenotypic characteristic with a myriad of possible combinations of parent and child eye colours. For example, two blue-eyed parents may produce a brown, green-hazel, or blue-eyed child. (1)

It is accepted that infant eye colour at birth does not necessarily correlate with resultant childhood and indeed adult eye colour, although it is found to be roughly stable by six years of age. (2) There is much anecdotal evidence of eye colour changing from one extreme of the spectrum to other in the first year of life. One study investigated this, finding a decrease in the number of light irises. (3) Congenital conditions affecting iris colour include albinism, in which the iris may be red or violet and heterochromia in which the two irises are different colours. (4)

The main contributory genes for iris colour are HERC2 (hect domain and RCC1-like domain-containing protein 2) and OCA2 (oculocutaneous albinism II) on chromosome 15, which account for around 74% of variation. Depression of OCA2 transcription results in a blue-eyed individual. A further 14 genes have been identified as contributing, however many of these play a greater role in determining skin and hair colour. Given the complex inheritance model of eye colour it is therefore difficult to predict final outcome based on the eye colour of the parents alone. (1, 4)

Many parents in the neonatal unit, ophthalmology and paediatric clinics enquire about changes in their child's eye colour, however literature is limited on the topic and the timing and nature of the change in eye colour in healthy neonates and young children is not well described.

ALSPAC is a vital resource in the study of child development and has contributed to much influential work on the topic of child development and paediatric disease. Simple data were collected regarding iris colour at various stages in the first 2 years of life and later at age 5. To our knowledge, this data has not been analysed to look at change in eye colour over time.

Understanding the nature of eye colour change in young children will broaden the knowledge base of both clinicians and patients or relatives and may aid in the understanding of iris development, which is important as it appears to be closely linked with brain development. (1) To date eye colour has not been found to be associated with visual acuity, but has been linked with reaction time, shyness, noise-induced hearing loss and ability to overcome seasonal affective disorder. (4) The individual iris is so unique in terms of colour, pattern and features that it can be reliably used in identification in place of a passport. (5)

The study team would be interested to investigate whether there was a change in iris colour between the age of 3 months and 5 years; at what point these changes occur and what form these changes take.

The study team are experienced in working with ALSPAC data and have paediatric ophthalmological clinical experience. Dr Creavin is an academic ophthalmology trainee whose research interest is ophthalmic epidemiology.

References

1. Sturm RA & Larsson M. Genetics of human iris colour and patterns. Pigment Cell Melanoma Res. 22;544-62

2. Bito LZ, Matheny A, Cruickshanks KJ, Nondahl DM, Carino OB. Eye color changes past early childhood: The Louisville Twin Study. Arch Ophthalmol 1997;115:659-63

3. Matheny AP and Brown Dolan A. Changes in eye colour during early childhood: sex and genetic differences. Annals of Human Biology 1975;2(2):191-6

4. White D and Rabago-Smith M. Genotype-phenotype associations and human eye color. Journal of Human Genetics 2011;56:5-7

5.Dougman J. Probing the uniqueness and randomness of IrisCodes: results from 200 billion iris pari comparisons. Proc. IEEE 2006;94:1927-35