The table below shows some details of three genetic conditions that affect the nervous and muscular systems in humans.
| Disease | Cause | Rate of cases | Effect |
|---|---|---|---|
| Becker muscular dystrophy | a recessive allele | 1 in 18,000 males | A mutation in the dystrophin gene leads to slowly progressive muscle weakness of the legs and pelvis. |
| Congenital myasthenic syndrome | a recessive allele | 1 in 200,000 people | A mutation prevents the functional synthesis of acetylcholinesterase, leading to prolonged depolarization at the neuromuscular junction. |
| Pelizaeus-Merzbacher disease (PMD) | a recessive allele | 3 in 125,000 people | A mutation prevents the proper formation of the myelin sheath around motor neurones. |
There are 75 million people in a country. Calculate the number of cases of PMD expected in this country.
Explain why PMD can cause difficulty in movement, using the information about its effect from the table.