The Science Behind Baby Predictions
OCA2/HERC2 two-gene eye color model, Punnett square implementation, Premium's three-generation analysis, and how the confidence score is computed up to its 95% cap.
Eye Color Genetics
Two genes, HERC2 and OCA2, solved as two Punnett squares and combined into six eye colors
OCA2 Gene (Primary)
OCA2 sets how much melanin the iris makes. It carries the P (pigment) and p alleles: two P's give brown or amber, one gives hazel or green, none gives blue. Chromosome 15.
HERC2 Gene (Regulator)
HERC2 switches OCA2 on or off, and it is the gene the blue-eye SNP sits in. A b/b genotype shuts OCA2 down for blue or gray eyes; a B allele lets OCA2's pigment through. Together the two genes produce six eye colors: brown, amber, hazel, green, gray and blue.
Punnett Square Engine
Each of the two eye-color genes is solved with its own 2×2 Punnett square (four outcomes each), then combined into six phenotypes. Hair color runs a dominance series across ten shades, with a separate MC1R square for red — and a parent's childhood hair color is treated as evidence of a hidden recessive allele.
Genotype to Phenotype Mapping
How gene combinations determine eye color
| HERC2 × OCA2 | Eye Color | Explanation |
|---|---|---|
| BB × PP | Brown | HERC2 fully active and both OCA2 alleles pigment-producing — maximum iris melanin. |
| Bb × PP | Amber | One HERC2 allele switched off, but full OCA2 pigment — the iris lands on amber. |
| BB × Pp | Hazel | Full HERC2 activity with only half the OCA2 pigment — the mixed hazel iris. |
| Bb × Pp | Green | Half the HERC2 activity and half the OCA2 pigment — green. |
| bb × P_ | Gray | HERC2 switched off, but OCA2 still carries a pigment allele — gray rather than pure blue. |
| __ × pp | Blue | No OCA2 pigment allele at all — the iris has no melanin to show, so blue. |
Polygenic Trait Models
Skin tone is polygenic; height we report as a mid-parental band rather than model from genes
Skin Tone (3 Genes)
An additive model over SLC24A5 (largest effect), SLC45A2 and TYR, producing a 0–100 melanin index reported with 68% and 95% confidence bands rather than a single value. Co-dominant blending inheritance.
Height (Mid-Parental Band)
The prediction engine reads no genotype for height. Each parent picks a height band; we average the two and report a range rather than a number — boys average +3 to +7 cm, girls average −3 to +1 cm. No standard deviation, no single figure: height is shaped by hundreds of variants plus nutrition and environment, so a band is the honest output.
Grandparent Evidence
Grandparent data constrains genotype possibilities. A blue-eyed grandparent proves the parent carries the recessive allele — and each extra generation you enter raises the completeness score behind the confidence figure.
Confidence Scoring
More family data means higher prediction confidence
| Data Available | Confidence | Explanation |
|---|---|---|
| Parents only | 60-70% | Basic Punnett square, no carrier knowledge |
| Parents + 2 grandparents | 90-95% | Partial genotype constraint |
| Parents + 4 grandparents | 95% (capped) | Strong genotype constraint |
| Parents + 4 grandparents + childhood hair color | 95% (capped) | Every ladder input present — the cap binds. |
Prediction Modes
Three modes offering different levels of data input and confidence
| Mode | Credits | Data Required | Generations |
|---|---|---|---|
| Scientific | 4 | Parent traits entered manually (no photos) | 2 (parents → baby) |
| Standard | 8 | Parent traits entered manually (no grandparents) | 2 (parents → baby) |
| Premium | 12 | Full parent traits + all 4 grandparents | 3 (grandparents included) |
Confidence is not set by the mode. It is one server-side score that starts at 60% and rises with the data you give us, hard-capped at 95%.
Scientific Validation
Built on peer-reviewed genetic research
Peer-Reviewed Research
Based on Sturm et al. (2008) and Eiberg et al. (2008) for the HERC2/OCA2 eye-color model, and Lamason et al. (2005) for SLC24A5 pigmentation. Height is not modeled from genotypes at all — it's a mid-parental band from the parents' own heights, and we say so rather than cite a GWAS we don't use.
Model Accuracy
Where the numbers come from: eye color is solved as two Punnett squares (HERC2 × OCA2); skin tone as a three-gene additive melanin index reported with 68% and 95% bands; height as a mid-parental band from the parents' own heights, not from genotypes. Every probability is computed in code before the AI writes a word — and the overall confidence score is hard-capped at 95%, because genetics is never certain.
Full Transparency
Every prediction includes confidence scores and Punnett square visualizations so users understand the science behind the results.
Frequently Asked Questions
Common questions about the genetics science behind ProbaBaby
Confidence isn't a mode setting — one score starts at 60% and rises with the data you give us, hard-capped at 95%. Parents only sits at 60%, reaching about 70% with ethnicity and both photos; all four grandparents plus childhood hair color reaches the cap.
If a parent has brown eyes, they could be BB (cannot pass blue) or Bb (can pass blue). A blue-eyed grandparent settles it: the parent must be Bb. That certainty is what raises your confidence score — any grandparent data adds 20 points and each individual grandparent another 5, up to the 95% cap.
OCA2 sets how much melanin the iris makes; HERC2 switches it on or off. We solve each as its own 2×2 Punnett square and combine them, which is why we can return amber, hazel, green and gray rather than just brown or blue.
Skin tone is polygenic. We model three of the best-characterised pigmentation genes — SLC24A5, SLC45A2 and TYR — as a weighted additive score on a 0–100 melanin index, and report it with 68% and 95% confidence bands rather than one number. Unlike eye color, skin tone follows co-dominant blending inheritance.
We report a range, not a point estimate. Each parent picks a height band rather than typing a measurement; the engine averages the two and adds a sex-specific offset — boys +3 to +7 cm, girls −3 to +1 cm. There's no standard deviation and no variance-explained figure behind it. Height is shaped by hundreds of genetic variants plus nutrition and environment, so treat the band as a rough guide, not a measurement. (The app's separate free Height tool uses the classic mid-parental ±6.5 cm formula — that's a different calculator, not the prediction engine.)
No. ProbaBaby uses simplified genetic models for educational and entertainment purposes. Predictions are probabilities, not guarantees. For diagnostic genetic testing, consult a certified genetic counselor.
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