As the X chromosome is one of the
sex chromosomes (the other being the
Y chromosome), X-linked inheritance is determined by the
sex 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. X-linked dominant disorders tend to affect females more often because they tend to be developmentally fatal in males. This is because males have only one copy of X-chromosome while females have two copies.
Inheritance 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 either sex have an even chance of receiving either of their mother's two X chromosomes, one of which contains the defective gene in question. 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. 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, or boys with
Klinefelter's syndrome (and hence have more than one X chromosome). A few scholars have suggested discontinuing the use of the terms
dominant and
recessive when referring to X-linked inheritance, stating that the highly variable penetrance of X-linked traits in females as a result of mechanisms such as
skewed X-inactivation or
somatic mosaicism is difficult to reconcile with standard definitions of dominance and recessiveness. ==List of dominant X-linked diseases==