Most X-linked conditions are recessive. This means that in a person with two X chromosomes (most females), both copies of a gene (i.e., one on each X chromosome) must have a change or mutation whereas in a person with one X chromosome (most males), only one copy of a gene must have a mutation. A female with a mutation in one copy of a gene on the X chromosome is said to be a “carrier” for an X-linked conditionWomen have two x chromosomes, compared to men who have one x and one y chromosome. Because of the two x chromosomes, one of them can override the other. In men they only have one x and one y, they don't have the extra x chromosome to override the allele.
