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.
39 exam-style questions on OCR GCSE Biology 3.1 Coordination and control – the nervous system, covering 3.1.1 Structure of the nervous system, 3.1.2 Coordinated response, 3.1.3 The reflex arc, 3.1.4 Structure of the eye, 3.1.5 Defects of the eye, 3.1.6 Structure and function of the brain, 3.1.7 Difficulties of investigating brain function, and 3.1.8 Limitations in treating nervous system damage. Each one has a worked solution and a mark scheme showing where the marks go.