In most mammalian species, females have two X sex chromosomes and males have an X and Y. As a result, mutations on the X chromosome in females may be masked by the presence of a normal allele on the second X. In contrast, a mutation on the X chromosome in males more often causes observable biological defects, as there is no normal X to compensate. Trait variations arising from mutations on the X chromosome are called “X-linked”.

One well-studied example of an X-linked trait is color blindness. When a mutation occurs in the genes responsible for red and green color vision in the photoreceptors of the retina, color blindness may occur. While this recessive mutation can cause females to be color blind, they must possess two mutated X chromosomes. Color blindness is much more common in males, who only have one X chromosome and therefore no second copy to potentially compensate for the mutation.

Color blindness is passed from mother to son; a mutated X will be passed from the mother to half of her sons, who receive a Y from their father. Meanwhile, colorblind males will pass on the allele for color blindness to all of their daughters, who will be either carriers or color blind, depending on the maternal allele received. Occasionally, this type of X-linked mutation can also arise by spontaneous mutation and not as the result of inheritance from mother or father.

Another well-studied example of an X-linked condition is hemophilia, a bleeding disorder well known for its high rate of incidence in European monarchies. Hemophilia is the result of a mutation in a blood clotting factor, either VIII or IV, that leads to longer bleeding after injury and can cause spontaneous joint bleeding. Though there is currently no cure for hemophilia, gene therapy is being explored as a possibility.