What is the probability of having a baby with heterochromia?

The baseline probability for any baby with no known family history is approximately 0.6%. If one parent has genetic heterochromia, this estimate rises to approximately 5.6%. If both parents are affected, the estimate rises to approximately 15.6%. The Heterochromia Probability Calculator uses a simplified additive model to estimate probability based on family history inputs.

Heterochromia Probability Calculator

Q: Is heterochromia hereditary?

Genetic (congenital) heterochromia can run in families but does not follow strict Mendelian dominant or recessive rules. The trait is often sporadic, appearing in children with no family history. When a parent has genetic heterochromia, the estimated probability in offspring is modestly higher than the population baseline. Acquired heterochromia caused by injury or medical conditions is not inherited and does not elevate risk to children.

Q: Does the Heterochromia Probability Calculator store my data?

No. All calculations run entirely in your browser using local JavaScript. We do not collect, store, or transmit any input values or personal data.

Estimate the likelihood your baby will be born with heterochromia based on parent and grandparent family history.

Calculate Your Baby's Heterochromia Probability

Does either parent have heterochromia?

Does any grandparent have heterochromia? (select "Don't know" if unsure — unknown grandparents count as zero)

Paternal grandfather (father's father)

Paternal grandmother (father's mother)

Maternal grandfather (mother's father)

Maternal grandmother (mother's mother)

Type of heterochromia in family (if known) (optional — informational only, does not affect probability)

Any known cause of the heterochromia in the family? (optional — important if the cause was injury or a condition)

What Is the Heterochromia Probability Calculator?

The Heterochromia Probability Calculator is a free tool that estimates the likelihood a baby will be born with heterochromia — a condition where the two eyes differ in color, or a single iris contains two distinct color zones. You enter information about family history: which parents and grandparents have heterochromia, the type of heterochromia observed in the family, and whether the known cause is genetic or acquired. The calculator returns an estimated probability percentage, a risk tier (Low, Slightly Elevated, or Elevated), and a brief explanatory note.

Heterochromia affects an estimated 0.6% to 1% of the global population — see how it compares to other eye colors in our Eye Color Rarity Calculator. For most families with no known history, the probability that any given baby will have heterochromia stays near this baseline. When one or both parents have the genetic form, the likelihood increases — but even in those cases, it remains well below certainty. This tool models that reality using a simplified additive approach, consistent with the non-Mendelian nature of heterochromia inheritance.

The output is an educational estimate, not a clinical or genetic diagnosis. It is intended to satisfy genuine curiosity about family trait inheritance. It does not replace genetic counseling or medical evaluation. For a broader eye color prediction that also considers heterochromia risk, try the Eye Color Calculator on this site. For a timeline of when baby eye color settles permanently, see our Baby Eye Color Change Timeline.

How to Use the Heterochromia Probability Calculator

Select parental history (required). Choose whether neither parent, the father only, the mother only, or both parents have heterochromia. This field has the largest effect on the probability estimate and must be completed to run the calculation.
Select grandparent history. For each of the four grandparents, choose Yes, No, or Don't know. If you are uncertain about a grandparent, select "Don't know" — unknown grandparents contribute zero to the estimate, so they will not inflate the result.
Select the type of heterochromia (optional). Choose Complete, Sectoral, Central, or Don't know. This field is informational and does not change the probability output. It is included to help you describe the family situation accurately.
Select the known cause (optional). If the heterochromia in the family is known to have been caused by injury, a medical condition, or medication, select "Caused by injury, medication, or condition." The calculator will then reset the probability to the baseline rate, because acquired heterochromia is not heritable.
Click "Calculate Probability." The tool displays the estimated probability, a risk tier badge, a visual meter showing where the estimate falls relative to the 35% maximum, an explanatory note, and a disclaimer.
Click "Reset" to start over. All fields return to their defaults and the result is cleared.

How Is the Heterochromia Probability Estimated?

The calculator uses a five-step additive model. Because heterochromia does not follow classic Mendelian dominant/recessive rules, the model uses risk-boost additions rather than formal probability multiplication. This approach is explicitly a simplified educational approximation, not a clinical genetic calculation.

**Calculation Steps and Risk Contributions**
Step	Factor	Probability Adjustment
1	Baseline (no family history)	0.6%
2	One parent has heterochromia	+5.0 percentage points
2	Both parents have heterochromia	+15.0 percentage points
3	Each grandparent with heterochromia	+1.5 pp (capped at +6 pp total)
4	Cause confirmed as acquired (injury or condition)	Resets to baseline 0.6%
5	Final cap	Maximum output: 35%

The risk tier is assigned from the final probability: Low (below 2%), Slightly Elevated (2% to 10%), or Elevated (above 10%). The cap of 35% prevents the model from implying near-certainty, which would be misleading given the sporadic nature of heterochromia — most children of two affected parents will still not have heterochromia.

If the known cause is confirmed as acquired, the grandparent and parental boosts are discarded and the probability resets to 0.6%. This reflects the biological reality that acquired iris pigmentation changes — caused by trauma, medication, or disease — do not alter the genetic information a parent transmits to a child.

What Is Heterochromia?

Heterochromia — formally called heterochromia iridis or heterochromia iridum — is a difference in coloration of the irises between the two eyes, or within a single iris. In most people, both irises are the same color. In individuals with heterochromia, there is a visible difference, ranging from a subtle inner ring of a different tone to a complete difference in color between the eyes.

Eye color is determined primarily by the concentration and distribution of melanin in the iris stroma. Brown eyes result from high eumelanin concentrations; blue and gray eyes emerge when melanin concentrations are low and light scattering through the iris structure creates cooler tones; green and hazel reflect intermediate concentrations combined with varying pigment distributions. Heterochromia occurs when this melanin distribution differs between the two eyes, or between the central and outer zones of a single iris, producing a noticeable color contrast.

The term "heterochromia" comes from the Greek words heteros (different) and chroma (color). The condition is most visible when the difference is large — for example, one brown eye and one blue eye. Milder forms, where only a small sector or inner ring of the iris differs in color, are more common and often unnoticed in daily life.

Types of Heterochromia

There are three main types of heterochromia, distinguished by the pattern and location of the color difference:

Complete heterochromia (heterochromia iridum): Each eye is a distinctly different color from the other. One eye may be brown and the other blue, green, or gray. This is the most visually striking and immediately recognizable form. It is what most people picture when they hear the word "heterochromia."
Sectoral heterochromia (partial heterochromia): A portion — or sector — of one iris is a different color from the rest of that same iris. The affected area can range from a small arc to a pie-slice-shaped patch. The other eye and most of the affected eye may otherwise be the same color. Sectoral heterochromia is more common than complete heterochromia and is often noticed only on close inspection.
Central heterochromia: A ring of a different color surrounds the pupil in one or both eyes, while the outer iris remains a separate color. For example, an iris that appears green at the outer edge but has a golden or amber ring around the central pupil. This is the most subtle form and is present in many people to a mild degree without ever being formally classified as heterochromia.

The type of heterochromia present in a family does not affect the probability calculation in this tool. Research has not established that one type is more or less heritable than another, so the type field is informational. A family with sectoral heterochromia and a family with complete heterochromia are treated identically in the probability model.

Is Heterochromia Hereditary?

Genetic (congenital) heterochromia — present from birth — can run in families, but its inheritance pattern is not straightforward. It does not follow simple Mendelian dominant or recessive rules the way many other traits do. Researchers believe that the genetics of iris pigmentation differences involve multiple genes and developmental processes, making the inheritance pattern complex and probabilistic rather than predictable.

In some families, heterochromia appears consistently across multiple generations. In others, it skips generations or appears in only one sibling out of several with the same parents. Sporadic occurrence — appearing in a child with no affected parent or grandparent — is common and well-documented. This means that even families with no history have a baseline probability (approximately 0.6%) of producing a child with heterochromia.

When a parent has confirmed genetic heterochromia, the estimated probability for a child is higher than the baseline but still well below 50%. When grandparents are affected without a parent being affected, the contribution is smaller, reflecting the reduced genetic proximity. Because the inheritance mechanism is complex, these estimates are approximations based on population-level observations rather than precise gene-level calculations.

Acquired heterochromia — heterochromia that develops after birth due to an external cause — is a completely separate situation. A parent whose heterochromia resulted from an injury, medication, or medical condition does not have altered iris-pigmentation genes to pass to children. Selecting the acquired cause option correctly reflects this biology by resetting the probability to the baseline rate.

What Causes Heterochromia?

Heterochromia can be congenital (present from birth) or acquired (developing later in life). The two categories have fundamentally different causes and different implications for heritability.

Congenital causes include natural genetic variation in melanin production and distribution across the two irises; Waardenburg syndrome, a rare genetic disorder involving mutations in genes such as PAX3, MITF, and SOX10, characterized by sensorineural hearing loss alongside pigmentation differences in the eyes, hair, and skin; congenital Horner syndrome, a neurological condition affecting the sympathetic nervous system that prevents normal melanin deposition in one iris; and piebaldism and other pigmentation disorders in which abnormal melanocyte migration during development creates localized color differences in the iris.

Acquired causes include eye injury or physical trauma that disrupts local melanin distribution in the affected iris; glaucoma medications — particularly prostaglandin analogue eye drops such as latanoprost — which can progressively darken the treated iris over months or years; chronic eye inflammation conditions such as iritis, uveitis, or Fuchs heterochromic iridocyclitis, which can lighten the affected iris over time; acquired Horner syndrome, caused by damage to the cervical sympathetic nerve chain through surgery, injury, or tumor growth; and, rarely, iris tumors or melanoma that alter the apparent color of a localized area.

Knowing the cause of heterochromia in the family matters for the probability estimate. If the cause is confirmed as acquired, the trait is not heritable and the probability resets to the population baseline. If the cause is genetic or unknown, the family history boosts apply as described in the calculation model.

Frequently Asked Questions

What is heterochromia?

Heterochromia is a condition where the two irises differ in color from each other (complete heterochromia), or where a single iris contains two distinct color zones (sectoral or central heterochromia). It affects an estimated 0.6% to 1% of the global population. The condition can be congenital — present from birth, often with a genetic basis — or acquired later in life as a result of injury, inflammation, medication side effects, or underlying medical conditions.

How common is heterochromia?

Heterochromia affects an estimated 0.6% to 1% of the global population, making it genuinely rare — rarer than green eyes, which occur in approximately 2% of people worldwide. Many cases of mild sectoral or central heterochromia go unnoticed in everyday life, so the actual prevalence of any degree of iris color variation may be somewhat higher than clinical estimates suggest. The figures in this calculator are based on population surveys that focus on noticeable heterochromia.

Is heterochromia hereditary?

Genetic (congenital) heterochromia can run in families, but does not follow strict Mendelian dominant or recessive rules. It is often sporadic, appearing in children with no family history of the trait. When a parent has genetic heterochromia, the estimated probability in offspring is modestly higher than the population baseline, but this is not a guarantee. Acquired heterochromia — caused by injury, medication side effects, or medical conditions — is not inherited and does not elevate the risk to children.

Can heterochromia skip a generation?

Yes. Because heterochromia does not follow strict Mendelian inheritance, the trait can appear to skip one or more generations. A grandparent may have heterochromia while none of their children have it, yet it reappears in a grandchild. This is why grandparent history is included in the calculator, though it carries less weight than parental history. The weighting reflects the reduced genetic proximity and the generally lower predictive value of grandparent history compared to parental history for complex, non-Mendelian traits.

What is the difference between complete, sectoral, and central heterochromia?

Complete heterochromia means each eye is a distinctly different color — for example, one brown eye and one blue or green eye. Sectoral heterochromia involves a patch, arc, or wedge of a different color within one iris, while most of that iris and the other eye remain the same color. Central heterochromia means a ring of one color surrounds the pupil while the outer iris is a different color — for example, a golden inner ring surrounded by a green outer iris. All three types can be congenital or acquired, and all three are covered by this calculator regardless of type.

If one parent has heterochromia, what are the chances my baby will have it?

If one parent has confirmed genetic heterochromia and no grandparent has it, the Heterochromia Probability Calculator estimates approximately 5.6% — compared to the 0.6% baseline for families with no history. This falls in the "Slightly Elevated" risk tier. The absolute probability is still low, and most children of one affected parent will not develop heterochromia. The trait is not predictable the way strictly Mendelian traits are, because it does not follow a clear dominant or recessive pattern.

If both parents have heterochromia, does that guarantee my baby will have it?

No. Even if both parents have genetic heterochromia, the calculator estimates approximately 15.6% — an Elevated risk compared to the population baseline, but still well below 20%. Heterochromia is not guaranteed or even probable even when both parents are affected, because the genetic basis is complex and not fully characterized. Most children born to two parents with heterochromia will have two matching eye colors. The probability cap of 35% in this model reflects that even extreme family history cannot make heterochromia near-certain.

Why does acquired heterochromia not count in the family history?

Acquired heterochromia develops after birth due to an external event — such as a physical injury to the eye, a medication side effect (some glaucoma eye drops can progressively darken one iris), or conditions like Fuchs heterochromic iridocyclitis. These causes do not alter the genetic information that a parent transmits to their children. Selecting "Caused by injury, medication, or condition" in this calculator correctly resets the probability to the 0.6% baseline, because the parent's heterochromia is not heritable in that case and family history has no bearing on the child's iris pigmentation genetics.

What does the "Low" risk tier mean?

A "Low" risk tier means the estimated probability is below 2% — close to the general population baseline of approximately 0.6%. This applies when neither parent has heterochromia, when all grandparents are unaffected, or when the known cause of heterochromia in the family is acquired rather than genetic. Most families who use this calculator will fall into the Low tier. The result does not mean it is impossible for the baby to have heterochromia — it simply means family history does not significantly raise the probability above the background rate.

What does "Slightly Elevated" risk mean?

A "Slightly Elevated" risk tier means the estimated probability is between 2% and 10%. This typically reflects one parent with genetic heterochromia, multiple grandparents with heterochromia but no affected parent, or a combination of grandparent-level family history. The absolute probability is still low — most families in this tier would not expect heterochromia — but it is meaningfully higher than the population baseline. The trait is not predictable enough to make heterochromia likely even at this tier.

What does "Elevated" risk mean?

An "Elevated" risk tier means the estimated probability is above 10%. This applies when both parents have heterochromia, or when one parent's history is combined with significant grandparent history. Even at this level, heterochromia in the baby is not the expected outcome — an Elevated probability of, say, 15% still means there is an 85% chance the baby will not have heterochromia. The "Elevated" label signals that family history meaningfully raises the odds, but it should not be interpreted as a prediction or near-certainty.

Can heterochromia develop after birth?

Yes. Acquired heterochromia can develop at any age. Causes include physical trauma to the eye, long-term use of certain glaucoma eye drops that stimulate melanin production in the treated iris, inflammatory eye conditions such as Fuchs heterochromic iridocyclitis that gradually depigment the affected iris, and acquired Horner syndrome caused by damage to the sympathetic nerve supply to one eye. Acquired heterochromia is distinct from congenital heterochromia in both cause and heritability — it does not affect the genetic risk for children.

Does heterochromia affect vision?

Congenital heterochromia alone does not typically affect vision. Most individuals with genetic heterochromia have entirely normal visual acuity in both eyes. The difference in iris pigmentation does not interfere with how the eye focuses light or processes visual information. However, heterochromia associated with certain underlying conditions — such as Horner syndrome, Waardenburg syndrome, or Fuchs heterochromic iridocyclitis — can be accompanied by vision-related symptoms specific to those conditions. If heterochromia develops suddenly, changes rapidly, or is accompanied by pain or visual changes, a medical evaluation is recommended.

What medical conditions are associated with heterochromia?

Several conditions can cause or be associated with heterochromia. Waardenburg syndrome is a genetic disorder involving mutations in genes such as PAX3 and MITF, characterized by heterochromia alongside sensorineural hearing loss and patches of different pigmentation in the skin or hair. Horner syndrome (congenital or acquired) involves disruption of the sympathetic nerve supply to one eye, resulting in that iris remaining lighter than usual. Fuchs heterochromic iridocyclitis is a chronic, low-grade inflammatory condition that progressively lightens one iris. Sturge-Weber syndrome and neurofibromatosis can also produce iris pigmentation changes. The presence of associated symptoms alongside heterochromia in a baby or child warrants evaluation by a physician or ophthalmologist.

Should I consult a doctor if my baby has heterochromia?

Yes, if a baby or child has heterochromia — particularly if it is newly noticed, appears to be changing, or is accompanied by other symptoms — a medical evaluation is advisable. Congenital heterochromia present from birth is usually benign, but a pediatric ophthalmologist can confirm whether the pigmentation difference is isolated or associated with an underlying condition. This calculator is an educational estimation tool and does not substitute for medical advice or professional evaluation.

How accurate is the Heterochromia Probability Calculator?

The calculator is an educational estimation tool, not a clinical or genetic risk assessment. It uses a simplified additive model grounded in general population patterns rather than individual genetic analysis. The true genetics of heterochromia are complex and not fully characterized, meaning any estimate carries significant uncertainty. Results should be understood as a broad indication based on family history, not a precise prediction. For a clinical risk assessment, consult a medical geneticist or genetic counselor.

Is the Heterochromia Probability Calculator free to use?

Yes, the Heterochromia Probability Calculator is completely free to use. No registration, account creation, subscription, or payment of any kind is required. Simply complete the form and click Calculate Probability to see your results instantly.

Do I need to sign up or create an account?

No. The Heterochromia Probability Calculator requires no registration or account of any kind. You do not need to provide an email address, create a profile, or agree to any terms beyond visiting the page. The tool is fully open-access with no barriers to use.

Does the Heterochromia Probability Calculator store my data?

No. All calculations run entirely within your browser using local JavaScript. The information you enter into the form is never transmitted to any server, stored in a database, or linked to any personal profile. We do not collect, share, or sell any personal data entered into the calculator. Your inputs are discarded as soon as you leave or refresh the page.

Can I use this calculator on my phone or tablet?

Yes. The Heterochromia Probability Calculator is fully responsive and works on smartphones, tablets, and desktop computers in any modern web browser. No app download or installation is required. The calculator form, result display, and all educational content are optimized for all screen sizes.

References

Wikipedia contributors. (2024). Heterochromia iridum. Wikipedia. https://en.wikipedia.org/wiki/Heterochromia_iridum
Cleveland Clinic Medical Professional. (2024). Heterochromia. Cleveland Clinic. https://my.clevelandclinic.org/health/symptoms/21715-heterochromia
American Academy of Ophthalmology. (2023). What is heterochromia? American Academy of Ophthalmology. https://www.aao.org/eye-health/diseases/what-is-heterochromia
MedlinePlus Genetics. (2024). Waardenburg syndrome. National Library of Medicine. https://medlineplus.gov/genetics/condition/waardenburg-syndrome/
Woerner, J., Bhardwaj, S., & Bhardwaj, P. (2020). Heterochromia iridis: A review of the etiology and management. International Ophthalmology Clinics, 60(2), 115–127. https://pubmed.ncbi.nlm.nih.gov/32109909/
Sturm, R. A., & Larsson, M. (2009). Genetics of human iris colour and patterns. Pigment Cell & Melanoma Research, 22(5), 544–562. https://doi.org/10.1111/j.1755-148X.2009.00606.x

Heterochromia Probability Calculator