19 Turn over 8 There are many reasons why humans may lose muscle mass. Two known causes are hip replacement surgery and some genetically inherited conditions. (a) A muscle fibre is a specialised body cell. Explain how the structure of a muscle fibre is related to its specialised function. 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(b) Muscles can be scanned using magnetic resonance imaging (MRI) to investigate the loss of muscle mass. It is safer to use MRI than computed tomography (CT) because (1) A CT uses X-rays that can cause mutations in the DNA of muscle fibres B CT uses X-rays that can cause mutations in the protein in the muscle fibres C CT uses magnets that can cause mutations in the DNA of muscle fibres D CT uses magnets that can cause mutations in the protein in the muscle fibres 20 (c) Muscular dystrophy (MD) is a range of genetically inherited conditions in which a loss of muscle mass can occur. Limb-girdle muscular dystrophy (LGMD2A) is a rare type of MD. Only six people per million have this recessive condition. In 2018 there were 66.5 million people in the UK. (i) Calculate the number of people in the UK who are carriers of LGMD2A using the Hardy-Weinberg equation. (3) p2 + 2pq + q2 = 1 Answer ............................................................... (ii) In the human population, the frequency of the allele for LGMD2A may change from one generation to the next. Explain why the frequency of this allele may change from one generation to the next. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 21 Turn over (d) Duchenne muscular dystrophy (DMD) is the most common inherited type of MD. DMD is caused by a recessive allele. The table shows some data on DMD from 1993 to 1999. Year Number of deaths due to DMD per year in England and Wales Median age of death / years males females 1993 75 0 19 1994 37 2 19 1995 41 0 20 1996 53 0 19 1997 43 1 20 1998 51 0 21 1999 51 1 20 Analyse the data to explain why DMD causes more deaths in males than in females. 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(Total for Question 8 = 14 marks) 22 BLANK PAGE