Color vision deficiency, commonly known as color blindness, impairs the ability to distinguish between certain colors, most often shades of red and green. For the vast majority of cases, this condition is sex-linked. This particular type of vision deficiency is one of the most well-known examples of a trait inherited through the sex chromosomes, which explains why the condition is observed much more frequently in one sex than the other.
The X Chromosome Connection
The genes responsible for producing the photopigments—light-sensitive proteins in the cone cells of the retina—are located on the X chromosome. Specifically, the opsin genes for the red-sensing (L) and green-sensing (M) cone pigments reside here. These genes follow a pattern of X-linked recessive inheritance, meaning the defective version of the gene must be present in a specific configuration to cause the condition.
The genetic makeup for sex differs: females possess two X chromosomes (XX), while males possess one X and one Y chromosome (XY). Because the color vision gene is on the X chromosome, the single X chromosome in males determines whether they will have the condition. If a male inherits an X chromosome carrying the defective, recessive gene, there is no corresponding gene on the Y chromosome to override it.
A female has two X chromosomes. If she inherits one X chromosome with the defective gene and one with the normal, dominant gene, the normal gene compensates. She will have standard color vision, but she is known as a carrier of the trait. For a female to be color blind, she must inherit the recessive gene on both of her X chromosomes, which is a statistically much rarer event.
Different Inheritance Scenarios
The way color blindness is passed down depends entirely on the genetic combination of the parents. A female who has one copy of the recessive gene and one normal copy is an unaffected carrier, able to pass the trait to her children. The transmission follows four primary scenarios, each with distinct probabilities for sons and daughters.
If a mother is a carrier and the father has normal vision, each son has a 50% chance of inheriting the affected X chromosome and being color blind. Each daughter has a 50% chance of inheriting the affected X chromosome and becoming a carrier herself, but a 0% chance of being color blind. The father cannot pass the X-linked trait to his sons because sons only inherit his Y chromosome.
In the scenario where the father is color blind and the mother has normal vision (not a carrier), all sons will have normal vision, as they receive the unaffected X chromosome from the mother. All daughters will automatically become carriers because they must inherit their father’s only X chromosome, which carries the defective gene. The most common way for a daughter to be color blind is if she has a color blind father and a mother who is at least a carrier.
Prevalence and Common Forms
The sex-linked nature of the most common deficiencies leads to a stark difference in prevalence between the sexes. Approximately 8% of males are affected by red-green color blindness, compared to only about 0.5% of females worldwide. This 16-to-1 ratio is a direct consequence of the X-linked recessive inheritance pattern.
The vast majority of inherited color vision deficiencies are of the red-green type, which includes Deuteranomaly (difficulty sensing green) and Protanomaly (difficulty sensing red). Both Deuteranomaly and Protanomaly are caused by mutations in the opsin genes on the X chromosome.
Rarer forms of color vision deficiency, such as Blue-Yellow color blindness (Tritanomaly or Tritanopia), are inherited differently. These types are caused by genes located on non-sex chromosomes, known as autosomes. Therefore, these blue-yellow deficiencies are not sex-linked and affect males and females with roughly equal frequency.