Disease or disorder that affects the nervous systems

There are many diseases/disorders that affect the nervous system, but here are a few:

Alzheimer's disease

Parkinson's disease

Epilepsy

Stroke

Explanation:

Alzheimer's disease: Alzheimer's disease is a progressive disease (gets worse and worse overtime) that destroys your memory. When a person has Alzheimer's disease, their neurons (nerve cells) get damaged and eventually die all throughout the brain. Some problems people with this disease can experience are: getting lost easily, personality/behavior changes, memory loss (obviously), and need help with self-care and daily things.

Parkinson's disease: Parkinson's disease is a progressive disorder that affects one's movement. Because the nerve cells are damaged, the dopamine (type of neurotransmitter) levels decrease. Since dopamine can control your body movement, it makes it hard to balance, and you're usually stiff. It usually begins with a tremor in your hand.

Epilepsy: Epilepsy is a neurological disorder where neuron activity in the brain gets disturbed, resulting in a seizure. Epilepsy is usually genetic or gained through a trauma or stroke (brain injuries). During these seizures, people often lose consciousness. Most epileptic people are sensitive to lights (flashing and flickering especially).

Stroke: A stroke occurs when your blood supply to your brain is interrupted/stopped. This is a SERIOUS emergency. Cells can die from lack of blood and the resulting lack of oxygen. Most warning signs of a stroke include: numbness + weakness, trouble speaking, loss of balance, and dizziness.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).