Genetic conditions with an X-linked recessive inheritance pattern usually affect only the boys in a family. This is because they are caused by altered or missing genes found on the X-chromosome. Girls are born with two X-chromosomes, so the effects of a working gene on one can mask the effects of an altered or missing gene on the other. They will normally be unaffected carriers of the condition. Boys have only one X-chromosome, so if they inherit one with an altered or missing gene, they will be affected.