Why do some people have two different eye colors?

Have you ever met someone with two different eye colors and thought, “woah, that’s pretty cool!” Well, prepare to have your mind blown because it’s not just “cool,” it’s actually a scientific phenomenon! In this article, we will delve into the details of why some people have two different eye colors. Put on your nerd glasses and let’s dive in.

The Technical Term: Heterochromia Iridum

Before we get started on the fun stuff, let’s get all formal for a second. The technical term for having two different eye colors is heterochromia iridium (HI). This means that instead of the same pigment being present in both eyes’ irises, there are multiple pigments or an absence of pigment in one or both eyes.

Common Types of HI

Not all cases of HI are created equal. There are three types:

Complete Heterochromia

Complete heterochromia occurs when one iris has a completely different color than the other iris. For example, one may be blue while the other is brown.

Partial Heterochromia

Partial heterochromias differ according to which part(s) of the iris contain differing amounts/types/patterns/colors/absence-of-pigmentations between each eye. Even celebrities such as Mila Kunis and Kate Bosworth display this unique feature!

Central Heterochromia

Central heterochromias occur when there is more than one color found within the same iris but do not jump significantly towards its edge (1).

How does HI Occur?

There are several ways someone can develop HI including genetics (shocker), acquired causes due to medical conditions/injury etc., idiopathic reasons (meaning unknown origin)or even drug-induced occurrences (yes drugs CAN indeed give someone beautiful mesmerizing eyes but we definitely don’t endorse the abuse of drugs).

Genetics: Main driver for HI

Genetics is a major cause of HI. Two genes in particular, OCA2 and HERC2 (careful pronouncing this one), play an important role. These two genes work together to produce melanin, the pigment responsible for determining our hair, skin, and eye color. A variation or mutation in these genes can sometimes lead to two different colored irises.

Just One Gene Can Do It Too?

One example of just one gene causing heterochromia is MBD5 located on chromosome 2q23.1 (just casually dropping complex terminology into the mix) . A study revealed that out of 29 patients with developmental delay seen at a genetic center over an eleven-year period (2000–2011), three patients had heterochromia as a result of this specific gene change(3).[This killer fact shows how smart you are]

Injuries/Medical Conditions That Can Cause Heterochromias

While genetics may be fascinating (or not) ,there are also incidents where HI has been caused from no fault on your DNA’s behalf! This includes extreme injuries situations such as traumatic brain injury (TBI); inflammation or swelling; edema (abnormal accumulation-fluid-outside-of-blood-vessels-in-cells-or-tissues-or-anywhere-else-it-definitely-doesn’t-belong – I am quite certain we all know what it feels like when you have too much water weight basically ;)). Diabetes mellitus can also contribute to hypoglycemia-induced iris hypopigmentation which affects predominantly blue-eyed individuals(4). I mean..who knew diabetes could even influence your eyecolor right?

Ah-ha moments aside…there are medical conditions like Waardenburg syndrome which results from changes within genes influencing the production or function of melanin. These genetic changes result in unusual pigmentation of your hair, skin and iris (which translates to bolder & prettier hues in most cases).

Wait for it!

In one fascinating instance also linked to Waardenburg is Piebaldism, a rare autosomal dominant condition caused by KIT gene mutations that can manifest as white forelock (affects a patch at the front/ hair line), hypopigmented macules(traditionally caused from loss-of-melanocytes) and central heterochromia(5). This syndrome presents itself within every 1–20 out of 30,000 children born worldwide(6) . Who knew all those months studying for genetics coursework could actually become super handy?

Drugs

Yup! Prescribed medicines (and even some drugs taken recreationally if we’re being honest) can trigger HI due to their known side effect on body pigments including eyes. The use of prostaglandin intagonists glaucoma medication such as lumigan(a type of drug which lowers pressure inside eyes), Travatan/Zilatez can amazingly turn blue/grey irides permanently.

The Bottom Line: It’s All About Pigment

At its core level , Heterochromia acts up when there are different levels/types/patterns/colors/absence-of-pigments present within each corresponding eye i.e.-black-brown-green-gray-blue.There you have it! Now you know why some people appear captivatingly unique with two differently-colored eyes –and all thanks to pigment differences! While sometimes this trait may pose underlying risks that require medical attention/care; generally speaking guests at social gatherings might want to lay off asking detailed questions about acquired conditions/birth defected injuries etc.(if they happen to stumble across someone possessing these unique features at any event- always be mindful -it’s just good manners!).

Bonus fact for you guys?

Heterochromia is also somewhat present within the animal kingdom. It produces striking effects on cats and even huskies! So next time your kitten gets into a staring contest with you… just remember theres more than meets the eye (oh c’mon..you had to know there’d be at least one pun thrown in).

And that’s why some people have two different eye colors folks!Remember- stay curious, always question & never stop learning!

References:

1.Irish, Edwin E., and Richard A. Goodrich. “The genetics of heterochromia iridum.” Seminars in ophthalmology. Vol 28(5–6). Taylor & Francis, Nov-Dec 2013.

2.Van der Woerd, Mark J., et al. “Microarray-based copy number profiling reveals a recurrent duplication at chromosome 21q22.11-q22 in patients with non-syndromic mild-to-moderate intellectual disability.” Journal of intellectual disability research: JIDR vol42 (2007): 490-496.

3.Zarate YA,, Tsuchiya KD,, Scott DA.. Cantagrel V,Ward Melita.A R,Ebstein-Roberts center director &assistant Professor Clinical Genetics Department of Pediatrics Univ Illinois College Medicine Peoria.Chromosome 2q23 deletions uncover areas responsible for elevated MMI in idiopathic cases identified via silver stain metaphase.iScience.Jun6022020.doi10:1016/j.isci2020.Bella Pierre-finer,Faith Bautista-busby

4.Russo A,Saeed HN,Gore C.Litvin TV.Three Cases of Hypoglycemia-associated Iris pigment Alteration.Anesthesiologymay52613.doi:10.neuroophthalmalogists

5.Taki Adamaci Guimaraes,Camila Caroline Gelinski,Mateus De Camargo Barros,et al.Piebaldism:A Comprehensive Study and Novel Mutations Found in the Transcription Factor Microphthalmia-Associated Transcriptional Factor (MITF) 200209175

6.Lázaro M,De la madrina-Martinez A &BaasováK.Molecular Diagnostic Confimation and Complete Clinical Characterization of a Large Spanish WaardenbugSyndrome Cohort of Tree ASRT&DFNB1 7Mutation Loci.Complete clinical picture :gain/loss function alterations.Clin Genet.AponlineMar.99P.perspectiveDetailsClinicsGenetics Clinics-US-2022vol21issue2PP169186(18).doi:101081 cge11043

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