10 4 Fertilisation in humans involves the fusion of a sperm cell with an egg cell. (a) (i) After the sperm cell nucleus fuses with the egg cell nucleus, the single fertilised cell is described as a (1) A diploid gamete B diploid zygote C haploid gamete D haploid zygote (ii) The fertilised cell then divides by (1) A meiosis to produce genetically different cells B meiosis to produce genetically identical cells C mitosis to produce genetically different cells D mitosis to produce genetically identical cells (b) Cell division of the fertilised cell produces a ball of totipotent cells. (i) Give the meaning of the term totipotent cell. 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Turn over 11 (ii) This ball of cells continues to divide to form the embryo. The cells of the embryo become specialised to form tissues and organs. Describe how cells become specialised. 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(c) Factors such as the diet of the mother during pregnancy can cause epigenetic changes in the embryo. Explain how epigenetic changes affect the development of tissues in the embryo. 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(Total for Question 4 = 10 marks)