2.i. Gene mutations in autosomes
a) Autosomal Dominant Inheritance
Diseases that have autosomal dominant inheritance affect individuals who have only one defective copy of the gene (from either parent).
|Autosomal Dominant Inheritance|
Huntington's disease (HD)
- Autosomal Dominant disorder which affects the central Nervous System (CNS)
- Patients with HD suffer involuntary movements such as twitching of the limbs and face and have slurred speech. As the disease progresses, they lose their intellectual abilities and have psychiatric disturbances which eventually lead to dementia and death.
- The defective gene responsible for HD is called Huntingtin and is located on the short (p) arm of chromosome 4 (4p). The gene contains a repeated CAG sequence near the 5’end.
- The mechanism that causes neuronal cell death in Huntington's disease may involve a hyperactive N-methyl-d-aspartate (NMDA) receptor. Because glutamate is the principal excitatory transmitter in the brain, most neurons have glutamate receptors. One of these receptors is called the NMDA receptor because it is selectively activated by the glutamate agonist NMDA.
- The mutation of the HD gene on 4p causes hyperactivity of the NMDA receptor. This hyperactivity causes excessive influx of calcium into the neurons of the caudate nucleus, and cell death occurs as a result.
- It is a form of dwarifism.
- The affected individual are small and disproportionate with abnormally short arms and legs
- Have unusually long or tapering fingers with poor musculature and long and thin extremities.
b) Autosomal Recessive Inheritance