Inheritance of X-Linked Recessive Genetic Diseases

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:

• Gender bias: Typically, males are the ones who get the disease, whereas females are carriers. Men cannot be carriers because they cannot have a bad X chromosome gene without getting the disease. Women cannot get the disease, because they typically have a second good X chromosome. However, in the rare case of a daughter of an affected father and a carrier mother, then the daughter might have two bad X genes and get the disease itself (like an autosomal recessive disease).
• Father to son transmission: 0% chance of disease and 100% chance of disease-free unless the mother is also a carrier (males always get their single X from the mother not father and cannot get a bad gene from the father), 0% chance of carrier (males cannot be carriers).
• Father to daughter transmission: 0% chance of disease (females can only be carriers), 100% chance the female child is a carrier (because the father gives a bad X gene as there is only the bad one to give). If the mother is also a carrier, the female can be fully afflicted with the rare double-recessive female version of the disease.
• Mother (carrier) to son transmission: 50% chance of disease, 50% chance disease-free, 0% chance of carrier (males cannot be carriers).
• Mother to daughter:: 0% chance of disease (females can only be carriers), 50% chance of female carrier, 50% chance neither affected nor carrier.
• Other children: If a couple has one (male) child with the disease, what are the odds for another child having it. Usually this means the mother is a carrier, because the father cannot transmit the disease to a child (and the father would probably have noticable disease). So the risk for a second child of the same couple is probably the mother-to-son transmission risk, 50% chance of disease, and a female child cannot have the disease but has a 50% chance of being a carrier.

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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