Inheritance and Genetics of X-linked Recessive Genetic Diseases

About inheritance and genetics:

Inheritance of X-linked Recessive Genetic Diseases refers to whether the condition is inherited from your parents or "runs" in families. The level of inheritance of a condition depends on how important genetics are to the disease. Strongly genetic diseases are usually inherited, partially genetic diseases are sometimes inherited, and non-genetic diseases are not inherited. For general information, see Introduction to Genetics.

Inheritance of X-linked Recessive Genetic Diseases: X-linked recessive diseases are typically passed from mother (who is a genetic carrier) on to a son (who gets disease), or to a daughter (who also becomes a carrier). However, rare cases of sporadic genetic disease can also occur. Not all children of a carrier mother will be affected. See details of inheritance patterns in article Inheritance of X-Linked Recessive Genetic Diseases.

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Inheritance properties of X-linked Recessive Genetic Diseases:   Disease inherited from: X-linked recessive diseases are typically inherited by sons from their mother, who is a symptom-free genetic carrier.

  Gender bias in inheritance: 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 with a bad gene without getting the disease. Women cannot usually get the x-linked recessive disease, because they typically have a second good X chromosome (except for very rare "double-dominance" form).

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Inheritance odds for X-linked Recessive Genetic Diseases:   Overall odds of inheritance: X-linked recessive diseases are typically inherited by sons from their mother, who is a symptom-free genetic carrier.

  Sibling of diseased child odds of inheriting disease: About 50%. If a couple has one (male) child with the x-linked recessive disease, usually this means the mother is a carrier, because the father cannot transmit the disease to any child (and the father would probably have noticable symptoms of having the x-linked recessive disease). So the risk for a second child of the same couple with x-linked recessive disease 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. In very rare cases of sporadic genetic disease, these odds do not apply.

  Mother to son inheritance odds: 50% chance of x-linked recessive disease, 50% chance disease-free, 0% chance of carrier (males cannot be carriers of x-linked recessive diseases, only females).

  Father to son inheritance odds: 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).

  Mother to daughter inheritance odds: 0% chance of disease (females can only be carriers of x-linked recessive diseases), 50% chance of female carrier, 50% chance neither affected nor carrier.

  Father to daughter inheritance odds: 0% chance of x-linked recessive 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.

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Genetic carriers of X-linked Recessive Genetic Diseases:   Carriers possible for disease?: Yes, all affected females are carriers of x-linked recessive diseases (unless they are double-dominant affected). Males can have the disease, but cannot be carriers.

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Inheritance features for X-linked Recessive Genetic Diseases:   Sporadic form of disease possible?: Yes, only a single genetic mutation is required on a gene in the X chromosome.

  Milder form of disease possible in females?: Yes, in some x-linked recessive diseases female carriers can have a mild form of disease rather than being fully symptom-free carriers. This occurs 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 female carriers. However, most X-linked recessive diseases have symptom-free female carriers.

  Double dominant form of disease possible?: Although not really "double dominant", females (who are XX) get a similar effect in the rare case where they get two bad X chromosomes. This can occur in the rare case of a daughter of an affected father and a carrier mother, then the daughter might have two bad X genes (similar to "double-dominance") and get the disease itself (like an autosomal recessive disease of the X chromosome for women-only). This is not true double-dominance because the disease is the same, whereas a double-dominant disease is typically more severe than a single dominant disease.

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