These conditions are caused by a mutation in a gene on the X chromosome, which is one of the sex chromosomes. Males have one X chromosome and one Y chromosome; females have two X chromosomes.
If a male’s X chromosome has the mutated gene, then he will have the condition. He has only one copy of the gene and the mutation isn’t on the Y chromosome.
If a female has the mutated gene on one of her X chromosomes, she still has a healthy copy of the gene on her other X chromosome. This makes her a ‘carrier’ and she will not usually have symptoms of the condition.
If a carrier of an X-linked condition and a non-carrier have children, there is a 50 percent chance that each son will have the condition and a 50 percent chance that each daughter will be a carrier (see first diagram).
This probability is the same for each child born. So, for instance, if this couple have two sons, each son will have a 50 percent chance of inheriting the condition.
Unaffected males cannot pass on an X-linked condition. Affected males cannot pass an X-linked condition to their sons, but all of their daughters will be carriers (see second diagram)