X-linked dominant

X-linked dominance is a pattern of inheritance involving a dominant allele that is carried on an X chromosome. X-linked dominant diseases include Charcot-Marie Tooth Disease (CMT).

The pedigree below shows a typical pattern of inheritance for an X-linked dominant disease:

Pedigree 5
Pedigree 5

There are many hallmarks of X-linked dominant diseases, notably:

  • More females are affected than males, due to the presence of an additional X chromosome in females, which increases the probability of inheriting the disease allele.

  • An affected mother's offspring, whether male or female, has a 50% chance of being affected, depending on which maternal X chromosome is inherited

  • An affected father will affect all of his daughters but none of his sons, since the daughter can only inherit the affected paternal X chromosome and the son can only inherit his father's Y chromosome

  • There is no male-to-male transmission, since only the Y chromosome is transmitted down the male line.

  • Males are usually more severely affected than females, because there is no compensation from a harmless allele as there is in females (males are hemizygous for X chromosome genes)