Biology 9700 · AS & A Level · Passage of information from parents to offspring
Passage of information from parents to offspring — practice question
In the brain, some neurones make the neurotransmitter dopamine. Parkinson’s disease develops when the neurones that make dopamine die. This disease usually appears in people older than 55 years. People younger than this who show these symptoms are described as having early onset Parkinson’s disease (EOPD). Recessive mutations in a gene called $\textit{PINK1}$, found on chromosome 1, an autosome, are thought to be one cause of EOPD. A person with this type of EOPD has a homozygous recessive genotype.
(a)[4]
Construct a genetic diagram for a cross between two individuals heterozygous for the $\textit{PINK1}$ gene locus. Add a key for the allele symbols, the parental genotypes, the gametes, the offspring genotypes, and the ratio of offspring phenotypes.
(b)[5]
$PINK1$ codes for a protein kinase enzyme that is important in how mitochondria function in neurones.
Most recessive $PINK1$ mutations are base substitutions that result in a non-functioning protein kinase enzyme.
Explain how a base substitution mutation can result in the production of a non-functioning protein kinase.
(c)[2]
One rare dominant mutation of the $PINK1$ gene produces a product that inhibits the normal protein kinase.
Explain how this mutation leads to EOPD in a heterozygote.
Worked solution & mark scheme
This 11-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Use $E$ for the normal allele and $e$ for the EOPD allele.” …