12   5 Porphyria is a life-threatening genetic disease. It is caused by a mutation in the gene coding for an enzyme involved in the production of haem. In people with porphyria, haem cannot be produced. (a) Explain why this mutation may prevent the enzyme involved in the production of haem from functioning correctly. 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Turn over 13   (b) Porphyria is caused by a dominant allele and may not develop until later in life. The pedigree diagram shows a family in which some individuals have porphyria. Key Affected male Affected female Unaffected male Unaffected female A B C (i) State the genotype and phenotype of person C. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (ii) Person A and Person B have one son with porphyria. What was the probability of them having a child that was male and had porphyria? (1) A 0.00 B 0.25 C 0.50 D 1.00 14   (c) A new technique known as gene silencing has been developed to treat this disease. Molecules known as small interfering RNA (siRNA) combine with mRNA to prevent translation. Deduce how siRNA may be used to prevent the development of porphyria. 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(Total for Question 5 = 10 marks) Turn over 15   6 Leigh syndrome is a disease of the nervous system that affects the brain. It is caused by a mutation in the DNA present inside mitochondria. (a) Draw and label a mitochondrion. (2) (b) Mitochondrial diseases such as Leigh syndrome are passed on by the mother during fertilisation. Explain why a fertilised egg cell will contain only maternal mitochondria. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 16   (c) Using IVF, it is possible to produce an embryo that does not have the mitochondrial disease. The nucleus is removed from a donor egg cell from another individual. The nucleus from a fertilised egg from a mother with the mutation is then placed in the egg cell from the donor. (i) Explain why the resulting embryo does not develop mitochondrial disease. 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(ii) Comment on the inheritance of parental characteristics in offspring produced in this way. 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(Total for Question 6 = 9 marks) Turn over 17   7 Measles is a contagious disease that can be controlled by vaccination. Not all parents choose to have their children vaccinated. The table shows the number of cases of measles and the percentage of children vaccinated in the UK between 2012 and 2017. Year Number of cases of measles Percentage of children vaccinated (%) 2012 1564 91.2 2013 1855 92.3 2014 135 92.7 2015 71 92.3 2016 556 91.9 2017 216 91.6 (a) (i) Calculate the percentage change in the number of cases of measles from 2013 to 2014. Give your answer to three significant figures. (2) ................................................................ % 18   (ii) One conclusion from the data is that it takes time for an increase in vaccination rate to reduce the number of cases of measles in children. Explain why this is a valid conclusion. 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Turn over 19   (b) The incidence of measles has risen by 300% worldwide in recent years. A study has found that some B memory cells and antibodies are destroyed by the measles virus. (i) Antibodies are released into the blood when (1) A B cells are activated to become killer cells B B cells are activated to become plasma cells C macrophages are activated to become B cells D plasma cells are activated to become memory cells. (ii) Vaccinations are carried out against many serious diseases. Vaccination leads to the production of antibodies and memory cells. Which kind of immunity is provided by this type of vaccination? (1) A active artificial B active natural C passive artificial D passive natural (iii) Explain why, following a measles infection, it may be advisable for children to repeat other vaccinations they have had. 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(Total for Question 7 = 11 marks)