St Leonard's College



ANSWERSPART A: MULTIPLE CHOICE QUESTIONSDDDABDBA CBPART B: SHORT ANSWER SECTION James Watson, Francis Crick (and Rosalind Franklin)[1]DNA molecule has the shape of a double helix2 strandscomposed of subunits called nucleotidesnucleotides contain phosphate, sugar and nitrogen basecovalent bonds hold the nucleotides together, forming the sugar-phosphate backboneHydrogen bonds hold the bases together across the centrecomplementary base pairing exits between the nitrogen bases (A=T, C=G)Explain DNA structure, including its subunits and the way in which they are bonded[4](Total 5 marks) locus[1]sex-linked[1]males only need one copy of the recessive allele to be colour blind (because they only have one X chromosome);females need two copies of the recessive allele to be colour-blind;[2] [3]xEeeEeeeeEeeeParents: Ee (mum) x ee (dad)Phenotype ratio: 50% round shaped blood : 50% elliptical shaped bloodWork out the possible phenotype for this child with regards to colour-blindness. Remember to use correct symbols and show all necessary working.[3]xXCXcXCXCXCXCXcYXC YXc YParents: XCXc (mum) x XCY (dad)Phenotype ratio: 50% normal females : 25% normal males : 25% colour blind malesKey (1 mark), correct structure (1 mark), genotypes (1 mark) [3]XcYXCY?XCXc???IIIIII(Total 13 marks)A heterozygote for a disorder – can pass it on without any symptoms themselves[1]2[1][3]xXHXhXHXHXHXHXhYXHYXhYKey: H = normal, h = haemParents: XHXh (mum) x XHY (dad)Phenotype ratio: 50% normal female : 25% normal male : 25% haemophiliac male (Total 5 marks) meiosis[1]Phases: 2 (meiosis) and 1 (mitosis) [3]Parent cell: reproductive cell (meiosis) and body cell (mitosis)Daughter cells: 4 (meiosis) and 2 (mitosis)Daughter cells: gametes (meiosis) and body cells (mitosis)Daughter cells: haploid (meiosis) and diploid (mitosis)Chromosomes in daughter cells: 23 (meiosis) and 46 (mitosis)Change in DNA sequence caused spontaneously or by environmental factors[1]Increased mutation increased variation[3][2]More variation allows for more competition and natural selection better suited organisms survive struggleSpecies begins to change over time as adaptations accumulate and increase in frequency(Total 6 marks) Divergent evolution[1]DNA analysis Comparative embryology or anatomyCheck if they can still mate to produce viable offspringAny other acceptable answer[2]Variation existedgeographical isolation occurred due to continental driftrestricted gene flow between two groups of snakesdifferent selection pressures acted on each group natural selection occursdifferent adaptations result in each group until they can no longer interbreed to produce viable offspring[4](Total 7 marks) chimpanzee[1]red kangaroo[1]number of differences = estimation of time since the two species shared a common ancestorchimpanzee and human share a more recent common ancestor than the red kangaroo and human[2](Total 4 marks) ................
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