|
|
|
X-linked inheritance |
|
|
The recessive, regular form |
This inheritance pattern only refers to genes that are present on the X chromosome.
Females have two X chromosomes and males have one X and one Y chromosome.
This is the reason why a male who carries an abnormal gene on his single X chromosome will express clinical
signs of disease whereas a woman who has an abnormal gene on only one of her X chromosomes will be a carrier.
A female carrier has a 50% chance in each pregnancy of transmitting her healthy X chromosome or her
abnormal X chromosome to the fetus. The 50% of the children (both sons and daughters) who receive the
normal X chromosome will all be healthy and not carriers. However, the 50% of the sons who receive the
abnormal X chromosome will all be affected, and the 50% of the daughters who receive this chromosome will all
be carriers like their mother.
An example of a disease transmitted by X-linked recessive inheritance is hemophilia A (a blood clotting disorder).
|
|
|
|
|
The dominant, rarer form
This inheritance pattern also only refers to genes that are present on the X chromosome,
but unlike in X-linked recessive inheritance (see above), carrier females will express some clinical signs.
Only a very small number of syndromes are transmitted in this manner. An example is vitamin D-resistant rickets.
|
|
|