Inheritance of X-Linked Dominant Diseases

These genetic diseases are diseases caused by an error in a single DNA gene. X-linked means the genetic error occurs in a gene on the 23rd sex-linked X chromosome. These diseases are sometimes called "sex-linked" rather than x-linked. This pair of chromosomes is different from all others because male and females are different: males have XY (one X and one Y chromosome), whereas females are XX (two copies of X and no Y). There are also autosomal dominant genetic diseases affecting chromosome 1..22 rather than the 23rd X chromosome.

Rare type of genetic disease: X-linked dominant genetic conditions are rarer than other types of genetic disease. Most X-linked genetic diseases are recessive. The majority of dominant genetic diseases affect the autosomal chromosomes 1..22. However, X-linked dominant genetic disease can occur.

Inheritance patterns for x-linked dominance: The patterns of genetic inheritance for X-linked dominant genetic diseases have the following features:

• No carriers: There is no such thing as a carrier in a dominant disease, because you cannot have the bad gene without having the disease.
• Father (XY) to Son (XY): 0% chance of disease (because the father's sperm provides only the Y chromosome, the son's good X comes from the mother's egg).
• Father (XY) to Daughter (XX): 100% chance of disease (because the father's sperm gives the daughter an X, and the XY father only has one X chromosome to give, i.e. the bad one).
• Mother (XX) to Son (XY): 50% chance of disease because the son gets one X chromosome from the mother, and has a 50% chance of getting the bad one versus the good gene.
• Mother (XX) to Daughter (XX): 50% chance of disease, because it depends on whether the child gets the good or bad X from the mother.
• Double dominant mother to child: The rare case of a mother with double-dominant disease (i.e. both X chromosomes have bad genes), has a 100% chance of passing disease to a child of either gender. Only females can be double-dominant.
• Gender bias: Both males and females can have the disease, since both can have an X chromosome with a bad gene that dominates. However, more females have the disease than men. This is because daughters have 100% inheritance from fathers combined with 50% from mothers, whereas sons only have 50% from mothers. Statistically, there should be 3 times as many women with the disease as men.
• Milder female disease: However, the female version of the disease can be milder for some diseases, because females have a second X chromosome, which should have a good gene that may mitigate somewhat against the dominant bad gene in the other X (i.e. perhaps the bad gene is not totally dominant).
• Double dominance: A female can have the double dominant form of the disease, with two bad copies of the gene. Double dominance can only occur if both the mother and father have the disease. This form is usually more severe than a dominant disease. (However it is unclear whether this is usually the same as the male version, who also has no good genes.)

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 dominant disease.

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