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The irises of human eyes exhibit a wide spectrum of colours. Eye color is a polygenic phenotypic trait determined by two factors: the pigmentation of the eye's iris [1] [2] and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris. [3]: 9
A petechia (/ p ɪ ˈ t iː k i ə /; [1] pl.: petechiae) is a small red or purple spot (≤4 mm in diameter) that can appear on the skin, conjunctiva, retina, and mucous membranes which is caused by haemorrhage of capillaries. [2] [3] The word is derived from Italian petecchia 'freckle', of obscure origin. [1]
The affected eye may be hyperpigmented (hyperchromic) or hypopigmented (hypochromic). [3] In humans, an increase of melanin production in the eyes indicates hyperplasia of the iris tissues, whereas a lack of melanin indicates hypoplasia. The term is derived from Ancient Greek: ἕτερος, héteros "different" and χρῶμα, chrôma "color ...
Any condition that causes the eyes to itch can contribute to darker circles due to rubbing or scratching the skin around them. Hay fever sufferers in particular will notice under-eye "smudges" during the height of the allergy season. Atopy can lead to frequent rubbing of the eyes, leading to local inflammation and increased pigmentation. [4]
Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene [5] and a G-protein-coupled receptor (GPCR). It is a light -sensitive receptor protein that triggers visual phototransduction in rods.
Redness of the sclera is typically caused by eye irritation causing blood vessels to expand, such as in conjunctivitis ("pink eye"). Episcleritis is a generally benign condition of the episclera causing eye redness. Scleritis is a serious inflammatory disease of the sclera causing redness of the sclera often progressing to purple.
Suddenly having dry eyes is likely related to an environmental trigger, although an underlying medical condition is also possible. Find answers here. What Causes Dry Eyes All of a Sudden Without a ...
Waardenburg syndrome is a group of rare genetic conditions characterised by at least some degree of congenital hearing loss and pigmentation deficiencies, which can include bright blue eyes (or one blue eye and one brown eye), a white forelock or patches of light skin.