Biology 9700 · AS & A Level · Passage of information from parents to offspring
Passage of information from parents to offspring — practice question
Huntington’s disease results from a dominant allele of the gene responsible for producing the huntingtin protein. This protein influences the development of a wide range of tissues, including brain tissue. The Huntington allele contains a number of repeats of the base sequence CAG, which codes for glutamine. As a result, the protein made includes a polyglutamine region. A gene with more than $39$ CAG repeats makes a protein that does not fold correctly and does not work properly. Symptoms of Huntington’s disease usually begin first between the ages of $30$ and $45$ years. There is no treatment for the disease, which gets worse over time and is always fatal. Some individuals with between $27$ and $35$ CAG repeats do not develop the disease, yet they may still pass the Huntington allele to their children, and these children may develop the disease because the number of repeats often increases when gametes are formed.
(a(i))[2]
The amino acid sequences on either side of the polyglutamine section of the huntingtin protein are unchanged despite the CAG repeats in the Huntington allele. Explain why this happens.
(a(ii))[4]
Using the information provided, explain why Huntington’s disease cannot be treated with gene therapy.
(b(i))[1]
State the probability that a young person with one parent affected by Huntington’s disease will inherit the Huntington allele.
(b(ii))[2]
Suggest one advantage and one disadvantage of screening for Huntington’s disease before any symptoms appear.
(c(i))[1]
State what the term embryo biopsy means.
(c(ii))[1]
Explain why PCR is used in this procedure.
(c(iii))[2]
Outline two social or ethical implications of screening embryos in this way.
Worked solution & mark scheme
This 13-mark question has a full step-by-step worked solution and mark scheme. One marking point: “CAG repeat is a triplet / triplets of bases/codons are inserted” …