As males have one X and one Y chromosome, when their X chromosome has a mutation the disease always manifests. Females have two X chromosomes, and when one of their X chromosomes has a mutation they are carriers of an X-linked disease. They may exhibit some symptoms of the disease or no symptoms at all, due to ‘X-inactivation’ – a mechanism that always ‘shuts off’ one of the two X chromosomes in females so they don’t have twice the number of genes as males, which would be toxic. As this process is random in the female cells, the X chromosome with the mutation can be silenced in varying degree in female carriers; thus explaining the symptom variability shown in X-linked diseases.