Genetic disorder

A genetic disorder, or genetic disease is a disease caused, at least in
part, by the genes of the person with the disease. There are a number of
possible causes for genetic defects:

   * They may be caused by an unwelcome mutation, as are most cancers.
   * There are genetic disorders caused by the accidental duplication of a
     chromosome, as in Down syndrome, or repeated duplication of part of a
     chromosome as in Fragile X syndrome.
   * The defective genes are often inherited from the person's parents. In
     this case, the genetic disorder is known as a hereditary disease. This
     can often happen unxpectedly when two healthy carriers of a defective
     recessive gene reproduce, but can also happen when the defective gene
     is dominant.

Genetic disorders can frequently be explained as due to the change of a
single DNA base in a gene, resulting in an enzyme or other protein either
not being produced, or having altered functionality. This can be trivial and
relatively harmless in its effects, such as color blindness, or lethal such
as Tay-Sachs. Other disorders, though harmful to those afflicted with them,
appear to offer some advantage to carriers; as in carriers of sickle cell
anemia and thalassaemia appearing to have enhanced resistance to malaria.

Several hereditary diseases are sex-linked, meaning that they afflict one
sex much more common than the other because the mutation is located on the X
(or, rarely, on the Y) chromosome.

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