X-linked dominant inheritance

X-linked dominant inheritance, sometimes referred to as X-linked dominance, is a mode of genetic inheritance by which a dominant gene is carried on the X chromosome. As an inheritance pattern, it is less common than the X-linked recessive type. In medicine, X-linked dominant inheritance indicates that a gene responsible for a genetic disorder is located on the X chromosome, and only one copy of the gene is sufficient to cause the disorder when inherited from a parent who has the disorder.

Genetics

As the X chromosome is one of the sex chromosomes (the other being the Y chromosome), X-linked inheritance is determined by the gender of the parent carrying a specific gene and can often seem complex. This is due to the fact that, typically, females have two copies of the X-chromosome, while males have only one copy. The difference between dominant and recessive inheritance patterns also plays a role in determining the chances of a child inheriting an X-linked disorder from their parentage.

Males are normally hemizygous for the X chromosome, having only one copy. As a result, X-linked dominant disorders usually show higher expressivity in males than females.

Inheritance

<br clear=all/> <br clear=all/> In X-linked dominant inheritance, when the mother alone is the carrier of a mutated, or defective gene associated with a disease or disorder; she herself will have the disorder. Her children will inherit the disorder as follows:

Of her daughters and sons: 50% will have the disorder, 50% will be completely unaffected. Children of both genders have an even chance of receiving either of their mother's two X chromosomes, one of which contains the defective gene in question.
<br clear=all/> <br clear=all/> When the father alone is the carrier of a defective gene associated with a disease or disorder, he too will have the disorder. His children will inherit the disorder as follows:

Of his daughters: 100% will have the disorder, since all of his daughters will receive one copy of his single X chromosome.
Of his sons: none will have the disorder; sons do not receive an X chromosome from their father.
<br clear=all/> <br clear=all/> If both parents were carriers of a defective gene associated with a disease or disorder, they would both have the disorder. Their children would inherit the disorder as follows:

Of their daughters: 100% will have the disorder, since all of the daughters will receive a copy of their father's X chromosome.
Of the sons: 50% will have the disorder, 50% will be completely unaffected. Sons have an equal chance of receiving either of their mother's X chromosomes.

In such a case, where both parents carry and thus are affected by an X-linked dominant disorder, the chance of a daughter receiving two copies of the X chromosome with the defective gene is 50%, since daughters receive one copy of the X chromosome from both parents. Were this to occur with an X-linked dominant disorder, that daughter would likely experience a more severe form.

Some X-linked dominant conditions such as Aicardi Syndrome are fatal to boys, therefore only girls with these conditions survive. Similarly, individuals with Klinefelter's Syndrome are referred to as "47,XXY Males".