X-linked conditions are conditions in which the defective gene (mutation) that causes the disorder is located on the X chromosome, which is one of the two sex chromosomes. Males have one X chromosome and one Y chromosome, while females have two X chromosomes and no Y chromosomes.
Example: Duchenne muscular dystrophy
If a condition is recessive, that means that it will not cause any (or only very mild) symptoms in people who have a normal copy of the gene as well as a defective one. Females have two X chromosomes, so unless they have a mutation in both of their X chromosomes (very rare) or some other rare problem that causes the normal copy not to fully compensate for the defective one, they will not show symptoms of the disease. But all females with a mutated gene are ‘carriers’, which means they can pass on the mutated copy of the gene to their children.
Males have only one X chromosome, so in their case one mutated copy of the gene in each cell is sufficient to cause the condition. This is why conditions like DMD affect only boys.
The image below shows the inheritance pattern for X-linked recessive disorders like DMD. Further information about genetics can be found in the links section to the left.