Traits that are determined by alleles carried on the X chromosome are referred to as X-linked. X-linked alleles require a specific notation: Xc or X+ where the “+” represents the dominant allele and the lowercase letter the recessive allele. Females will have two X-linked alleles (because females are XX), whereas males will only have one X-linked allele (because males are XY). Most X-linked traits in humans are recessive.
One example of an X-linked trait is red-green colorblindness. Let (Xc) represent the recessive allele that causes colorblindness and (X+) represent the normal dominant allele. Females that are X+X+ or X+Xc have normal color vision, while XcXc females are colorblind. Males that are X+Y have normal color vision, while XcY males are colorblind.
To determine the inheritance of red-green colorblindness (or any other X-linked trait), the genotypes of the parents must be considered.For example, if a mother is a carrier for colorblindness (X+Xc), and a father has normal vision X+Y, then their sons have a 50% chance of colorblindness because they inherit their X chromosome from their mother and their Y chromosome from their father. Their daughters will have a 50% chance of being a carrier (X+Xc) and a 50% chance of being completely normal (X+X+) (see figure). A Punnett square can be used to determine any possible genotypic combinations in the parents.
Here is a pedigree depicting X-linked recessive inheritance. These traits are often passed from a carrier mother to an affected son. X-linked traits are never passed from father to son. Males are more likely to be affected than females. In this pedigree, the carrier (heterozygous) females are indicated; however, they do not express the trait being tracked in this pedigree.