What decides hair colour
Hair colour comes from two pigments. Eumelanin is brown to black, and pheomelanin is red to yellow. Lots of eumelanin gives black or brown hair, and very little gives blond. When both copies of a gene called MC1R carry certain changes, the hair cells switch to making mostly pheomelanin, and the hair is almost always red.
Many genes set the exact amounts, so real hair colour is a smooth range of shades. The calculator keeps the two effects that matter most for the question parents ask: dark or light, and red or not.
The two-gene model
- A pigment gene, dark or light. Dark is dominant. Two dark copies, or one dark and one light, give black or brown hair. Two light copies give blond hair.
- MC1R, red or not red. Red is recessive. Two red copies give red hair, whatever the pigment gene says. One red copy shows no red, but the parent can pass it on.
Each biological parent passes one copy of each gene to the child at random, and the calculator adds up every combination.
The assumptions about hidden genes
You cannot see which hidden versions a parent carries, so the calculator uses stated, rough averages:
- Light pigment: 4 in 10 pigment copies in the population are assumed to be light, which makes about 16% of people blond. A dark-haired parent then carries a hidden light copy 4 times in 7 and passes it on about 29% of the time. Blond is much rarer in southern Europe and more common in the north, so this is a middle value.
- Red: a parent without red hair is assumed to carry the red version 1 time in 3. That fits the UK, where 4.6% of adults have natural red hair (UK Biobank). If someone in their family has red hair, the calculator raises this to 1 in 2. It would be certain if the parent's own mother or father has red hair, and lower if it is a distant cousin.
- A red-haired parent: red hides the pigment gene, so the calculator uses the population average for it.
A worked example
A red-haired parent and a brown-haired parent with a red-haired sister: the red-haired parent always passes on red, and the brown-haired parent does so 1 time in 4 under the family assumption. So the child has a 25% chance of red hair.
For the other 75%, blond needs a light pigment copy from both parents: 40% from the red-haired parent times about 29% from the brown-haired one, so about 11%. That gives about 9% blond and about 66% black or brown.
Typical results without red hair in the family
| Parents | Black or brown | Blond | Red |
|---|---|---|---|
| Dark and dark | 91% | 8% | 1% |
| Dark and blond | 71% | 28% | 1% |
| Blond and blond | 0% | 99% | 1% |
| Red and dark | 80% | 10% | 10% |
| Red and blond | 54% | 36% | 10% |
| Red and red | 0% | 0% | 100% |
Hair changes as children grow
The hair a baby is born with often falls out in the first months and grows back in a different shade. After that, light hair commonly darkens through childhood, and again around puberty, possibly because some pigment genes become more active with age. Plenty of blond toddlers become brown-haired adults, so the calculator describes the likely adult colour, not the colour in baby photos.
Limits of the model
This is a simplified model, and real inheritance is more complex. Dozens of genes nudge the amount of each pigment, so two blond parents can have a brown-haired child, and black and brown cannot be told apart here. The carrier figures are averages, not facts about your family. Grey and white hair, which come with age, are not part of it. Treat the percentages as a fun estimate, not a genetic test.