All males have one X-chromosome, and all females have two X-chromosomes. Males get Lesch-Nyhan disease when there is a mutation of the hprt gene on the X-chromosome. Females do not get the disease because the normal X-chromosome protects them. These females are known as silent carriers, because they carry the disease, but it remains silent.
Females who are carriers can pass the good chromosome or the bad chromosome to their children. If they pass the good one along, their children are healthy. If they pass the bad one along, the result depends on whether they have a boy or a girl. If the bad chromosome is passed along to a daughter, the daughter is normal but becomes a silent carrier like her mother. If the bad chromosome is passed to a son, he gets Lesch-Nyhan disease. Because every boy inherits his X-chromosome from his mother, only mothers can transmit the disease. Fathers cannot pass the disease to their sons.
Lesch-Nyhan boys do not always get their condition from their mothers. In some cases, their mothers do not carry a bad gene, and they are normal. These boys got their disease when a mutation in the hprt gene occurred very early during their development. Genetic testing is required to determine if the mutation came from the mother or not.