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          The general rule of X-linked genetic diseases is that they are passed from mother to son, hence the reason they are "X-linked."  Another general rule is that carriers are not affected by the disease because genes on their unaffected X chromosome compensate for the damaged genes on the affected X chromosome.

          However, spontaneous mutations may cause X-linked genetic diseases to appear in  family that has no history of them and no carriers.  These spontaneous mutations can also cause females to be affected by the diseases.  Since females have two copies of all genes on the X chromosomes, half of them are eventually inactivated.  Generally, affected genes are inactivated and healthy ones prevail.  If the healthy copy of a gene is inactivated, a female may be affected by an X-linked genetic disease.

          And, finally, it is possible, but very rare for an individual to have the diseased, affected genotype and a completely normal phenotype.  There is really no explanation for this, but it has been known to occur.

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Fragile-X Syndrome

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Duchenne Muscular Dystrophy

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