X-linked dominant
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X-linked dominant is mode of inheritance in which a gene on the X chromosome is dominant.[1] Females can be more frequently affected than males since they have two X chromosomes that could potentially carry the abnormal gene, whereas a male has only one. However, the Lyon hypothesis states that X-inactivation renders only one copy of the X chromosome active in each cell hence on average one would expect only one half of the cells to express the abnormal gene. The chance of passing on an X-linked dominant disorder differs between men and women.
This inheritance pattern is less common than X-linked recessive.
[edit] Inheritance
Females usually have two X chromosomes, while most males have one X and one Y chromosome. If a child has inherited the mutation from the X chromosome of one of their parents they will have the condition. A woman with an X-linked dominant disorder has a 50% chance of having an affected daughter or son with each pregnancy. The sons of a man with an X-linked dominant disorder will not be affected (since they inherit their only X chromosome from their mother), but his daughters will all inherit the condition.
 A woman with an X-linked dominant disorder has a 50% chance of having an affected child. |
 The daughters of a man with an X-linked dominant disorder will all inherit the condition. |
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".
