Answer from: Maria José Mendiola, MD, MSc in Human Genetics, MSc in Science Communication, ObGyn
Variants on the x chromosome are most often recessive. Some cases they could be dominant but in most cases they are recessive so, this means that if there is a working copy of the same gene, in each cell of a person, they will not usually show symptoms of the condition. In other words, the working copy of the gene is a backup for the other non-working copy of the gene. We have two copies and, for example, we have a girl, a female that has two copies of a gene because they are two X chromosomes: one of the X chromosomes has a recessive variant and the other has a working copy of the gene. In this case, this person is a genetic carrier and although only one of the gene copies is sending the right instructions to make the gene product, it can usually work with this. In many x-linked recessive conditions, females who carry a gene variant will not have any symptoms of the conditions. Some conditions however may affect females who are genetic carriers and this is thought to be to the X inactivation process that switches off the x with the working gene and more body cells so, the working copy of the x chromosome is not switched on enough to protect this person from developing the conditions. Males who have in this case the recessive gene variant on their x chromosome do not have a second chromosome with a working copy of the gene – they do not have a backup therefore, a they will not able to send the right message to the cells to make the gene product and they will have the condition.