Huntington's disease. Friedreich's ataxia. Friedreich's ataxia is an autosomal recessive inherited disease that causes progressive damage to the nervous system, resulting in symptoms ranging from gait disturbance to speech problems; it can also lead to heart disease and diabetes, but does not affect thinking ability.
The ataxia of Friedreich's ataxia results from the degeneration of nerve tissue in the spinal cord, in particular sensory neurons essential (through connections with the cerebellum) for directing muscle movement of the arms and legs. The spinal cord becomes thinner and nerve cells lose some of their myelin sheath (the insulating covering on some nerve cells that helps conduct nerve impulses). Myotonic dystrophy. It is characterized by wasting of the muscles (muscular dystrophy), cataracts, heart conduction defects, endocrine changes, and myotonia.
Two types of myotonic dystrophy exist. Myotonic dystrophy type 1 (DM1), also called Steinert disease, has a severe congenital form and an adult-onset form. Myotonic dystrophy type 2 (DM2), also called proximal myotonic myopathy (PROMM) is rarer than DM1 and generally manifests with milder signs and symptoms. Myotonic dystrophy can occur in patients of any age. Both forms of the disease display an autosomal dominant pattern of inheritance. Fragile X syndrome. Fragile X syndrome (FXS), also known as Martin–Bell syndrome, or Escalante's syndrome (more commonly used in South American countries), is a genetic syndrome that is the most widespread single-gene cause of autism and inherited cause of intellectual disability especially among boys.
It results in a spectrum of intellectual disabilities ranging from mild to severe as well as physical characteristics such as an elongated face, large or protruding ears, and large testes (macroorchidism), and behavioral characteristics such as stereotypic movements (e.g. hand-flapping), and social anxiety. Fragile X syndrome is associated with the expansion of the CGG trinucleotide repeat affecting the Fragile X mental retardation 1 (FMR1) gene on the X chromosome, resulting in a failure to express the fragile X mental retardation protein (FMRP), which is required for normal neural development.