X-linked is a trait where a gene is located on the X chromosome. Humans and other mammals have two sex chromosomes, the X and the Y. In an X-linked or sex linked disease, it is usually males that are affected because they have a single copy of the X chromosome that carries the mutation. In females, the effect of the mutation may be masked by the second healthy copy of the X chromosome.
X-linked refers to genes that are on the X chromosome. So the X chromosome is one of the two sex chromosomes that we have in humans. Females have two X chromosomes, and males have an X and a Y chromosome. Most genes on the X chromosome, when mutated, only show a phenotype in males. And that's because, since females have two X chromosomes, the chromosome that doesn't have a mutation can often compensate for the other X chromosome if it has a mutation. Some X-linked conditions that individuals may be familiar with are Fragile X syndrome, hemophilia A, and Duchenne muscular dystrophy. But there are some conditions that are more common in females, such as Rett syndrome, which is also due to mutation on the X chromosome.- Suzanne Hart, Ph.D.