Some neurones in the brain produce a neurotransmitter known as dopamine. Parkinson's disease occurs when the neurones that produce dopamine die. A person with the disease may experience difficulty in coordinating movement, often seen as tremors (shaking) in different parts of the body. Parkinson's disease typically occurs in people older than 55 years. Younger people with these symptoms are said to have early onset Parkinson's disease (EOPD). Recessive mutations in a gene known as PINK1, located on chromosome 1, an autosome, are believed to be one cause of EOPD. A person with this form of EOPD has a homozygous recessive genotype.

1. (a)Draw a genetic diagram of a cross between two individuals who are heterozygous at the PINK1 gene locus. key to symbols used for alleles parental genotypes gametes offspring genotypes ratio of offspring phenotypes[4]
2. (b)PINK1 codes for a protein kinase enzyme that is important in the functioning of mitochondria in neurones. Most recessive PINK1 mutations are base substitutions which lead to the production of a non-functioning protein kinase enzyme. Explain how a base substitution mutation can lead to the production of a non-functioning protein kinase.[5]
3. (c)One rare, dominant mutation of the PINK1 gene codes for a product that inhibits the normal protein kinase. Explain how this mutation causes EOPD in a heterozygote.[2]