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These genetic diseases are diseases caused by an error in a single DNA gene. X-linked means the error occurs on the X chromosome which is the 23rd sex-linked X chromosome. Such diseases are sometimes called "sex-linked" rather than X-linked.
Some examples of X-linked recessive disorders are Hemophilia and Duchenne muscular dystrophy. These occurs only in boys, which is what we expected from an X-linked recessive disorder, as discussed below. For a list of this type of disorders, see x-linked recessive disorders.
Recessive means that disease only occurs when a person has two copies of the bad gene. For autosomal recessive diseases, this usually means they must inherit the disease from both parents, but this is not the case for X-linked recessive diseases. Men have only a single X chromosome, so they have only one copy of any gene on the X chromosome. Thus, a gene error on X definitely caused disease in men (who are XY), but women are XX, and have two copies of the gene. X-linked recessive disorders are more likely to occur than autosomal recessive disorders, because men have only one X chromosome, whereas all people have 2 copies of each autosome.
Recessive diseases often occur in genes that produce an enzyme. In a carrier, who has only one bad copy, there is often no disease, because the second gene can pull up the slack, and maintain health. In some recessive diseases, a carrier gets a mild form of the disease. For example, in X-linked recessive hemophilia, a female carrier has one bad gene on chromosome X, but the good gene on the other X chromosome produces enough of the good clotting enzyme to maintain health. The recessive disease only arises when the male has no good gene on the other chromosome (because they get a Y instead of a second good X).
X-linked recessive inheritance: These diseases arise from an error on the X chromosome, which causes disease only when there is no corresponding paired X chromosome with a good gene. However, since men are XY a man with the bad gene on the X chromosome must get the disease, because there is no second X chromosome. Since women are XX, they usually have a second good X chromosome which suppresses the bad X gene, leaving them disease-free, but as carriers. The following patterns of inheritance are typical:
Mild disease in female carriers: Female carriers can have a mild form of disease, because they have a bad gene on one of their two X chromosomes, and a good gene on the other. If the disease is not totally recessive, a partial disease can result even though the woman has one good gene. In other words, if the second gene copy is not a good enough "backup", a partial level of mild disease can still result in carriers. However, most X-linked recessive diseases have symptom-free female carriers.
Sporadic cases: A genetic disease that occurs when neither parent has any genetic defect is called a sporadic genetic disease. These cases arise via random genetic mutations in the DNA. A sporadic mutation can be the cause of an x-linked recessive disease (whereas it is unlikely for an autosomal recessive disorder) because only a single mutation is required (in males) to cause the disease. Females can also become carriers owing to random mutations.
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