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X-linked inheritance
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The recessive, regular form
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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).
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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.
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