Baby Genetics Calculator

Diagram of human iris colors including brown, hazel, green, blue, and gray eye variations — Figure 1. Human iris colour ranges from brown through hazel, green, and blue to grey. The primary genetic determinant is the HERC2/OCA2 locus on chromosome 15, which regulates melanin production in the iris stroma. Higher melanin produces darker eye colours; lower melanin allows structural (Rayleigh) scattering to produce blue and grey appearances.

The Genetics of Eye Colour — Beyond Simple Dominance

Eye colour is one of the most misunderstood topics in genetics education. The old textbook model — brown is dominant, blue is recessive, two blue-eyed parents always have blue-eyed children — is a simplification that does not reflect the actual biology.

Eye colour is a polygenic trait. At least 16 genes contribute to iris pigmentation. The primary genes are HERC2 and OCA2 on chromosome 15. A single nucleotide polymorphism (SNP) in intron 86 of the HERC2 gene (rs12913832) regulates transcription of OCA2. The OCA2 protein, also known as the P protein, influences melanosome maturation and the amount of eumelanin (brown/black pigment) produced in iris melanocytes.

Brown eyes have high eumelanin concentration in the anterior iris stroma. Blue eyes have very low eumelanin — the blue appearance results from Rayleigh scattering of light by the stromal collagen fibres, the same optical phenomenon that makes the sky blue. Green and hazel eyes have intermediate eumelanin levels plus varying amounts of pheomelanin (yellow/red pigment).

Brown eyes — most common globally

High eumelanin in the anterior iris stroma. Strongly associated with homozygous brown alleles at HERC2/OCA2. Dominant in populations of African, South Asian, and East Asian ancestry. Estimated global prevalence: 55–79%.

Blue eyes — low melanin, structural colour

Near-absent eumelanin in the stroma. The blue colour is structural — caused by Rayleigh scattering, not blue pigment. Strongly linked to the rs12913832 T allele in HERC2. Most common in Northern and Eastern European populations.

Green eyes — rarest common colour

Low-to-intermediate eumelanin with variable pheomelanin. Results from specific combinations at HERC2/OCA2, SLC24A4, and other loci. More common in Central and Northern Europe. Estimated global prevalence: 2%.

Hazel eyes — complex polygenic expression

Heterogeneous iris with variable melanin distribution — often brown near the pupil with green or gold peripherally. Reflects intermediate allele combinations across multiple pigmentation loci. Prevalence varies widely by population.

Hair Colour Inheritance — MC1R, Eumelanin, and Pheomelanin

Hair colour is determined by the ratio and total amount of two types of melanin produced in hair follicle melanocytes: eumelanin (black/brown) and pheomelanin (red/yellow). Multiple genes regulate melanin synthesis and deposition, including MC1R, TYRP1, SLC45A2, OCA2, and KITLG.

The MC1R gene (melanocortin 1 receptor) on chromosome 16 is particularly important for red hair. Loss-of-function variants in MC1R shift melanin synthesis from eumelanin to pheomelanin, producing the red/auburn phenotype. MC1R variants are co-dominant — a single variant allele often produces reddish tints or freckling even in dark-haired individuals who do not appear classically red-haired.

Dark hair (black and dark brown) results from high eumelanin with minimal pheomelanin, governed by alleles at ASIP, TYRP1, and SLC45A2. Blonde hair involves low eumelanin and low pheomelanin, associated with specific alleles at KITLG and SLC24A4. The published genome-wide association study by Sulem et al. identified six primary hair colour loci explaining the majority of variation in European populations.

Baby Blood Type Inheritance — ABO Codominance and Rh Factor

Unlike eye and hair colour, ABO blood type follows simple Mendelian inheritance with three alleles: I^A (type A), I^B (type B), and i (type O). I^A and I^B are codominant — a person with both alleles (I^AI^B) has blood type AB, expressing both A and B antigens simultaneously. The i allele is recessive to both.

Rh factor (Rhesus factor) is determined primarily by the RHD gene. Rh-positive (Rh+) is dominant over Rh-negative (Rh-). Two Rh-positive parents who are both heterozygous carriers (Rr) have a 25% chance of producing an Rh-negative child. Rh incompatibility between an Rh-negative mother and an Rh-positive foetus is clinically significant — the mother may develop anti-D antibodies that can affect future pregnancies. This is managed with anti-D immunoglobulin.

Parent 1 Blood Type	Parent 2 Blood Type	Possible Baby Blood Types
O	O	O only
A	O	A or O
B	O	B or O
A	A	A or O
B	B	B or O
A	B	A, B, AB, or O
O	AB	A or B only
AB	AB	A, B, or AB (never O)

For detailed blood type probability calculations, use our dedicated Blood Type Calculator with full ABO and Rh factor Punnett Square analysis.

Why Trait Predictions Are Probabilities, Not Guarantees

Two parents with identical observable traits can carry different underlying genotypes. Two brown-eyed parents may both carry one copy of blue-associated alleles at HERC2/OCA2 — creating a small but real chance of a blue-eyed child. Two parents with the same hair colour may carry different combinations of MC1R, TYRP1, and KITLG alleles, leading to unexpected outcomes in their children.

Environmental factors also modulate gene expression. Hair colour commonly darkens with age as melanocyte activity increases. Iris colour in infants often shifts during the first 6–18 months of life as melanin accumulates in the stroma. A baby born with blue eyes may develop brown or hazel eyes by their second birthday.

For precise genetic prediction, whole-genome sequencing or genotyping arrays (such as those used in direct-to-consumer DNA tests) can identify the actual alleles at each pigmentation locus. These results provide far more accurate predictions than phenotype-based estimates. Our calculator uses population-level probability tables that reflect the best available evidence from published genetics studies.

Frequently Asked Questions — Baby Genetics

What eye colour will my baby have?

Your baby's eye colour depends primarily on variants in the HERC2 and OCA2 genes on chromosome 15, with contributions from at least 14 other genes. Brown eyes result from high melanin in the iris stroma and are most common worldwide. Two blue-eyed parents have approximately a 78% chance of a blue-eyed baby, but can occasionally produce green or hazel-eyed children due to complex polygenic interactions. Use the calculator above to estimate probabilities based on both parents' eye colours.

Can two brown-eyed parents have a blue-eyed baby?

Yes, though it is uncommon — estimated at around 1% probability. Both brown-eyed parents would need to carry recessive alleles at the HERC2/OCA2 locus. The simplified dominant/recessive model often taught in schools overstates the certainty; eye colour is genuinely polygenic with multiple genes contributing to iris pigmentation levels.

How is blood type inherited from parents?

ABO blood type follows standard Mendelian inheritance with three alleles: I^A, I^B (codominant), and i (recessive). Each person inherits one allele from each parent. Blood type A results from genotype I^A I^A or I^A i. Blood type B from I^B I^B or I^B i. Blood type AB from I^A I^B. Blood type O from ii. Rh factor (positive/negative) is determined by the RHD gene, with Rh+ dominant over Rh-.

Can two blonde parents have a dark-haired baby?

It is very unlikely but not impossible. Hair colour is polygenic, involving multiple genes including MC1R, TYRP1, SLC45A2, and OCA2. Two blonde parents carry mostly alleles associated with low melanin production. A dark-haired child from two blonde parents would require several pigmentation alleles aligning in an unusual combination — the probability is very low, estimated at under 2%.

How accurate is a baby genetics calculator?

Baby genetics calculators give probabilistic estimates, not guarantees. Eye and hair colour are polygenic — influenced by many genes we cannot fully specify without DNA sequencing. Blood type prediction is more accurate because it follows Mendelian inheritance with known alleles. These tools are educational and useful for understanding inheritance patterns, but should not be used for clinical decisions.

What genes determine eye colour?

The primary eye colour genes are HERC2 and OCA2 on chromosome 15. HERC2 contains a regulatory region that controls OCA2 expression — the OCA2 protein affects melanin production in the iris. A single nucleotide polymorphism (SNP) in intron 86 of HERC2 (rs12913832) is strongly associated with blue versus brown eye colour and explains about 74% of eye colour variance. Additional contributing genes include SLC24A4, TYR, TYRP1, SLC45A2, and MITF.

Do fathers or mothers have more influence on baby's eye colour?

Neither parent consistently has more influence on eye colour. Both parents contribute one allele at each gene locus to the child. The HERC2/OCA2 SNP is autosomal (not sex-linked), so paternal and maternal contributions are equal. However, because brown alleles are more common globally and show something close to dominance over blue alleles, a brown-eyed parent statistically increases the probability of brown-eyed offspring regardless of which parent has brown eyes.