TRIPLET REPEAT MUTATIONS
Fragile X
syndrome
is due to triplet nucleotide repeat mutations, so that the
nucleo-tide sequence CGG repeats typically hundreds to thousands of times. The
mutation occurs in the FMR-1 gene
(fragile X mental retardation-1) on the X chromosome (Xq27.3), and the disease
behaves as an X-linked dominant disease that causes intel-lectual disability in
all affected males and 50% of female carriers. The characteristic phenotype
includes elongated face with a large jaw, large everted ears, and
macroor-chidism. The condition can be diagnosed with DNA probe analysis.
Huntington
disease is due to a triplet repeat mutation (CAG) of theHTTgene thatproduces an abnormal protein
(huntingtin), which is neurotoxic and causes atrophy of the caudate nucleus.
Huntington disease has an early onset (age range: 20–50 years) of progressive
dementia with choreiform movements.
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