How Color Blindness Is Inherited — X-Linked Inheritance and Odds Table
Why red-green color vision deficiency is far more common in men, the odds for children by parent combination, carriers, and how blue-yellow defects and achromatopsia are inherited differently.
The L and M photopigment genes (OPN1LW, OPN1MW) behind red-green color vision deficiency (protan and deutan) sit on the X chromosome. Men have one X chromosome and women have two — and that difference drives the difference in frequency between the sexes.
Why it's more common in men
A man has only the one X he received from his mother, so if that X carries a variant, he has a color vision deficiency. A woman shows the trait only if both of her X chromosomes carry it, which is far rarer. Among people of European ancestry, red-green deficiency affects about 8% of men and about 0.4–0.5% of women.
Squaring the male frequency gives roughly the female frequency (0.08 × 0.08 ≈ 0.0064). The actual female rate is a bit lower because a woman with a protan variant on one X and a deutan variant on the other usually has normal color vision — each X fills in what the other lacks.
Odds for children by parent combination
The table below assumes the parents carry red-green deficiency genes of the same type. A “carrier” is a woman who has one X with the variant but usually has normal color vision.
| Father | Mother | Sons | Daughters |
|---|---|---|---|
| Normal | Carrier | 50% affected · 50% normal | 50% carriers · 50% normal |
| Affected | Normal (non-carrier) | All normal | All carriers |
| Affected | Carrier | 50% affected · 50% normal | 50% affected · 50% carriers |
| Normal | Affected | All affected | All carriers |
| Affected | Affected | All affected | All affected |
A father's color vision deficiency is never passed directly to his sons, because a son's X always comes from his mother.
That's why color vision deficiency often seems to skip a generation: “maternal grandfather → mother (carrier) → son.”
Can carriers be identified?
Most carrier women test normal on standard tests. Some show very subtle differences in color discrimination, and in extremely rare cases there has been evidence of tetrachromacy, the use of a fourth color dimension (Jordan et al., 2010). Family history is the most reliable clue, and genetic testing can confirm it if needed.
Gene arrays and color weakness
On the X chromosome, one L gene is followed by one or more M genes lined up in a row. Because these very similar genes sit side by side, misaligned crossing over during egg or sperm formation easily causes genes to be lost or creates hybrid genes that mix L and M. This is considered the reason red-green color vision deficiency is more common than other inherited traits.
Inheritance of other types
- Tritan (blue-yellow): the S photopigment gene (OPN1SW) is on chromosome 7 and is inherited in an autosomal dominant pattern. Even if only one parent has it, each child has a 50% chance of inheriting it, with no difference between the sexes. Severity, however, varies widely even with the same variant.
- Achromatopsia (rod monochromacy): autosomal recessive. When both parents are carriers, each child has a 25% chance of having achromatopsia.
- Blue cone monochromacy: X-linked recessive, so it follows the same table as red-green deficiency and mostly affects men.
Good to know
- If one brother has red-green color vision deficiency, the mother is likely a carrier, and each other son also has a 50% chance.
- Acquired color vision deficiency (from eye disease, medication, or aging) is not inherited.
- For genetic counseling, contact an ophthalmologist or a genetic counseling clinic. This table is a simplification meant to aid understanding.