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Unformatted text preview: al U.S. population. The recessive allele (and the inherited disease) is significantly more frequent
in Ashkenazi jews of eastern european origin. For this group the incidence of the disease = 1/3,000. The
heterozygous condition can be detected by genetic screening, so genetic counseling is available.
***Xeroderma Pigmentosum (278700, for example): This disease can be caused by recessive mutations in at
least seven different genes, all involved in repairing damaged DNA. U.V. light on the exposed skin of such persons
results in painful burns and blisters, then skin cancers and early death. This inherited abnormality shows the
importance of the DNA repair system in the normal functioning cell and in the prevention of cancer. It also
highlights concerns over increased exposure to U.V. light by normal individuals.
Achondroplasia (100800): This allele affects the growth centers of the long bones. The affected individual has a
normal sized torso, but very short limbs. The homozygous condition is lethal and naturally aborted. There is no
treatment. Frequency = 1/10,000 live births.
Brachydactyly (112500): Results from the reduction or elimination of the middle bone of the fingers, so that all of
the fingers look like thumbs. This mutation is also lethal when homozygous. Brachydactyly has the distinction of
being the first human genetic disorder shown to be inherited in a dominant Mendelian fashion (circa 1903).
***Huntington Disease (143100): A rare instance of a lethal dominant. Death is preceded by symptoms of
increasingly severe and uncontrolled palsy (shaking) and dementia. The allele continues to exist in the population
because the onset of the disease is usually in the person's mid forties, after most child bearing has been completed. The gene for this disease has now been cloned, and testing for the heterozygous condition is poss...
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